Fix savegame on 64bit

This makes the saving/loading work on my 64bit Linux.

Every time I tried to load a save made by the game, this error occurred:
ERROR: Loaded chunk ID (GHL2) does not match requested chunk ID (GCLI)

I tracked the problem's origin down here: https://github.com/jakubadler/OpenJK/blob/cbf692cb1aa35eabccb38a12f694ea58dc951cec/code/game/g_savegame.cpp#L984
There is a `-2` cast to `int` stored in the file, which isn't the same as `-2` cast to pointer, so this condition doesn't evaluate true when it should, hence the missing chunk.
So I made the `GetGClientPtr` and `GetGClientNum` functions to work with `inptr_t` instead of `int`. I also modified the `GetGEntityNum` and `GetGEntityPtr` the same way, because they also stored pointers as `int`, though they didn't seem to cause problems.