m!m's fix for Mantis #3131: contrary to the xt change from r11138, NULL is only returned on vm_malloc failure (so should never happen in normal execution)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11179 387891d4-d844-0410-90c0-e4c51a9137d3