Check for presence of buddy in info/dinfo handlers

The res_jabber resource module uses the ASTOBJ library for managing its ref
counted objects.  After calling ASTOBJ_CONTAINER_FIND to locate a buddy object,
the pointer to the object has to be checked to see if the buddy existed.
Prior to this patch, the buddy object was not checked for NULL; with this patch
in both aji_client_info_handler and aji_dinfo_handler the pointer is checked
before used and, if no buddy object was found, the handlers return an error
code.

This patch does not take the approach that our JID can be used to log in from
another resource.  If that approach is desired, an improvement could be made to
this patch to create the buddy on the fly.  This patch seeks only to prevent
Asterisk from crashing.

FYI: In Asterisk 11+, you really should be using res_xmpp.  It does not have
this problem, as it moved to the astobj2 library.

Note that multiple people have proposed patches for this issue; the patch being
committed here is based on those.

(closes issue ASTERISK-19532)
Reported by: Karsten Wemheuer
Tested by: Byron Clark
patches:
  fix-jabber uploaded by Karsten Wemheuer (license #5930)
  xmpp_no_crash_with_ejabberd.patch uploaded by Byron Clark (license #6157)

(closes issue ASTERISK-19557)
Reported by: ulugutz
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Merged revisions 374335 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374336 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374337 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374338 65c4cc65-6c06-0410-ace0-fbb531ad65f3