res_fax.c: crash on framehook with no dsp in fax detect

In fax_detect_framehook() a null pointer reference can occur where a
voice frame is processed but no dsp is attached to the fax detection
structure.  The code block that rejects frames that detection cannot
be processed on is checking for dsp but falls through when it should
instead return, as this change implements.

(closes issue ASTERISK-22942)
Reported by: adomjan
Review: https://reviewboard.asterisk.org/r/3076/
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Merged revisions 404351 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 404352 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404353 65c4cc65-6c06-0410-ace0-fbb531ad65f3