main/file.c: fix possible extra ast_module_unref to format modules.

fn_wrapper only adds a reference to the format's module if the file
was able to be opened.  If not this causes an unmatched
ast_module_unref in filestream_destructor.  Move ast_module_ref to
get_stream.

ASTERISK-24492 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4149/
........

Merged revisions 427464 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427465 f38db490-d61c-443f-a65b-d21fe96a405b