roll back r10806 because 1) all callers to this function have names which are MAX_FILENAME_LEN in size; and 2) the memset already sets all characters to 0, so there's no need to add a null terminator

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10857 387891d4-d844-0410-90c0-e4c51a9137d3
fix a bug introduced by r10837: 0 is a valid path index

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10858 387891d4-d844-0410-90c0-e4c51a9137d3