qcommon: fix potential buffer overflow in COM_StripFilename

[Exagone313]

Fixes a potential buffer overflow and a compilation warning (warning: ‘strncpy’ specified bound depends on the length of the source argument [-Wstringop-overflow=]).
The COM_StripFilename function is only used once on a MAX_QPATH sized char array.
Also saw that Q_strncpyz function uses strncpy with len-1 and ensures a NUL at len-1 (so it is safe).

[ensiform]

Probably better to rewrite it with buffer size in parameter list and possibly use strrchr for \ and / depending on platform or if its a qpath then you can do / only.

[Exagone313]

Updated prototype of COM_StripFilename for outsize in first commit, and changed COM_SkipPath to use strrchr in the second (rebased just in case).