Add `CSnapshot::IsValid` to check if a snapshot unpacked from a snapshot delta or demo is valid:
- ensure number of items and data size are not negative
- ensure that the actual size of the snapshot matches the size derived from its member variables
- ensure item offsets are within the valid range
- ensure item sizes are not negative
Add `CSnapshot::TotalSize` and `CSnapshot::OffsetSize` utility functions.
Minor improvements to related error messages.
Fixes buffer overflow:
```
==47744==ERROR: AddressSanitizer: global-buffer-overflow on address 0x558618e3767f at pc 0x558614b9bdfb bp 0x7ffe58a32cd0 sp 0x7ffe58a32cc0
READ of size 4 at 0x558618e3767f thread T0
0x558614b9bdfa in CSnapshotItem::Type() const src/engine/shared/snapshot.h:16
0x558615c3c911 in CSnapshot::GetItemType(int) const src/engine/shared/snapshot.cpp:29
0x558614aebaba in CClient::UnpackAndValidateSnapshot(CSnapshot*, CSnapshot*) src/engine/client/client.cpp:2264
0x558614af87cb in CClient::OnDemoPlayerSnapshot(void*, int) src/engine/client/client.cpp:2598
0x558615b9db1a in CDemoPlayer::DoTick() src/engine/shared/demo.cpp:659
0x558615babd3f in CDemoPlayer::Update(bool) src/engine/shared/demo.cpp:1007
0x558614afb08b in CClient::Update() src/engine/client/client.cpp:2686
0x558614b1d9eb in CClient::Run() src/engine/client/client.cpp:3296
0x558614b8e64f in main src/engine/client/client.cpp:4761
```
And fixes a buffer overflow that manifests itself as an internal ASan error:
```
=================================================================
==4755==AddressSanitizer CHECK failed: ../../../../src/libsanitizer/asan/asan_descriptions.cc:79 "((0 && "Address is not in memory and not in shadow?")) != (0)" (0x0, 0x0)
0x7f0bf5f368be in AsanCheckFailed ../../../../src/libsanitizer/asan/asan_rtl.cc:72
0x7f0bf5f54eee in __sanitizer::CheckFailed(char const*, int, char const*, unsigned long long, unsigned long long) ../../../../src/libsanitizer/sanitizer_common/sanitizer_termination.cc:77
0x7f0bf5e4cb6f in GetShadowKind ../../../../src/libsanitizer/asan/asan_descriptions.cc:79
0x7f0bf5e4cb6f in __asan::GetShadowAddressInformation(unsigned long, __asan::ShadowAddressDescription*) ../../../../src/libsanitizer/asan/asan_descriptions.cc:95
0x7f0bf5e4cb6f in __asan::GetShadowAddressInformation(unsigned long, __asan::ShadowAddressDescription*) ../../../../src/libsanitizer/asan/asan_descriptions.cc:92
0x7f0bf5e4e386 in __asan::AddressDescription::AddressDescription(unsigned long, unsigned long, bool) ../../../../src/libsanitizer/asan/asan_descriptions.cc:440
0x7f0bf5e50e94 in __asan::ErrorGeneric::ErrorGeneric(unsigned int, unsigned long, unsigned long, unsigned long, unsigned long, bool, unsigned long) ../../../../src/libsanitizer/asan/asan_errors.cc:380
0x7f0bf5f35f4d in __asan::ReportGenericError(unsigned long, unsigned long, unsigned long, unsigned long, bool, unsigned long, unsigned int, bool) ../../../../src/libsanitizer/asan/asan_report.cc:460
0x7f0bf5e86f5e in __interceptor_memset ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:762
0x558234873f1d in mem_zero src/base/system.cpp:213
0x55823481fc27 in CSnapshotBuilder::NewItem(int, int, int) src/engine/shared/snapshot.cpp:675
0x55823481be65 in CSnapshotDelta::UnpackDelta(CSnapshot*, CSnapshot*, void const*, int) src/engine/shared/snapshot.cpp:380
0x558234776641 in CDemoPlayer::DoTick() src/engine/shared/demo.cpp:631
0x5582347861a9 in CDemoPlayer::Update(bool) src/engine/shared/demo.cpp:1007
0x5582336d4c7d in CClient::Update() src/engine/client/client.cpp:2695
0x5582336f75dd in CClient::Run() src/engine/client/client.cpp:3305
0x558233768241 in main src/engine/client/client.cpp:4770
```
src/engine/shared/snapshot.cpp:72:8: runtime error: signed integer overflow: -1297193910 + -1824658838 cannot be represented in type 'int'
#0 0x565165b0687d in CSnapshot::Crc() /media/ddnet/src/engine/shared/snapshot.cpp:72:8
#1 0x565165c4d20b in CClient::ProcessServerPacket(CNetChunk*) /media/ddnet/src/engine/client/client.cpp:1981:49
#2 0x565165c5d960 in CClient::PumpNetwork() /media/ddnet/src/engine/client/client.cpp:2589:6
#3 0x565165c67a71 in CClient::Update() /media/ddnet/src/engine/client/client.cpp:2856:2
#4 0x565165c72f4e in CClient::Run() /media/ddnet/src/engine/client/client.cpp:3237:4
#5 0x565165c94b7e in main /media/ddnet/src/engine/client/client.cpp:4341:11
#6 0x7fba5af2c151 in __libc_start_main (/usr/lib/libc.so.6+0x28151)
#7 0x5651659e1e0d in _start (/media/ddnet/DDNet+0x705e0d)
Purely automatic change. In case of conflict with this change, apply the
other change and rerun the formatting to restore it:
$ python scripts/fix_style.py
teehistorian records all inputs from the players as well as the player
positions in each tick. It stores this info in a highly compressible
output format (I've achived 5x compression using xz or bz2).
This system can easily be extended by independent authors without
collisions, something the old system with plain increasing integers did
not allow.
Do this by utilizing the previously unused message code `NETMSG_NULL`
which has a value of 0.
This works for engine and game messages, snapshot items and events.
