cli.c: Fix tab completion of "module load" when MALLOC_DEBUG is enabled.

filename_completion_function() returns memory that was not allocated by
the MALLOC_DEBUG allocation tracker so the memory must be freed by
ast_std_free().


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@421600 f38db490-d61c-443f-a65b-d21fe96a405b