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/
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Merged revisions 427464 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 427465 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 427466 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@427467 65c4cc65-6c06-0410-ace0-fbb531ad65f3