6035: Fix various issues reported by cppcheck static analyser r=def- a=Robyt3
After generating `compile_commands.json` with cmake, I ran [cppcheck](https://cppcheck.sourceforge.io/) like this:
```
cppcheck --project=compile_commands.json -DWIN64 --suppressions-list=cppcheck.supp --enable=all 2>cppcheck.log
```
With these suppressions in `cppcheck.supp`:
```
cstyleCast
useStlAlgorithm
unusedFunction
variableScope
noExplicitConstructor
useInitializationList
noConstructor
uninitMemberVar
uninitMemberVarPrivate
uninitDerivedMemberVar
uninitStructMember
uninitvar
shadowFunction
memleakOnRealloc
internalAstError
virtualCallInConstructor
unknownMacro
noOperatorEq
noCopyConstructor
```
Many of these occur too often or are false positives.
Here is a list of all remaining non-suppressed issues reported by cppcheck: [cppcheck.log](https://github.com/ddnet/ddnet/files/9997663/cppcheck.log)
And here is a list of all remaining issues including the suppressed ones: [cppcheck_all.log](https://github.com/ddnet/ddnet/files/9997662/cppcheck_all.log)
I couldn't get cppcheck's command line argument to ignore the external folders to work correctly, so I manually removed those entries from the files.
## Checklist
- [ ] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)
Co-authored-by: Robert Müller <robytemueller@gmail.com>