3987: Fix client use-after-free reported by mmmds r=def- a=ChillerDragon
Fix https://github.com/teeworlds/teeworlds/issues/2645
The `Sv_ReadyToEnter` net message is only valid while we're still
loading stuff, it is used to signal that we can send the `ENTERGAME`
system message. Simply ignoring the server message while we're in the
wrong part of the state machine works.
(cherry picked from commit a97f8a22de)
## 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 if it works standalone, system.c especially
- [ ] 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: heinrich5991 <heinrich5991@gmail.com>
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