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().
........

Merged revisions 421600 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@421602 65c4cc65-6c06-0410-ace0-fbb531ad65f3