iax2-provision: Fix improper return on failed cache retrieval

(closes issue ASTERISK-20337)
reported by: John Covert
Patches:
    iax2-provision.c.patch uploaded by John Covert (license 5512)
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Merged revisions 373342 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373343 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@373368 65c4cc65-6c06-0410-ace0-fbb531ad65f3