Remove a tautological Assert(), which cannot be triggered due to the type

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10866 387891d4-d844-0410-90c0-e4c51a9137d3
Fix Clang warnings about incorrect type given different pointer sizes for the architecture. Use the standard compliant ptrdiff_t

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10867 387891d4-d844-0410-90c0-e4c51a9137d3