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().
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Merged revisions 421600 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 421602 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@421608 f38db490-d61c-443f-a65b-d21fe96a405b