res_pjsip: crash when using localnet and external_signaling_address options

There was a collision of mod_data use on the transaction between using a nat
hook and an session response callback.  During state change it was assumed
what was in the mod_data was nothing or the response callback.  However, it
was possible for it to also contain a nat hook thus resulting in a bad cast
and a crash.

Added the ability to store multiple data elements in mod_data via a hash table.
In this instance, mod_data now stores a hash table of the two values that can
be retrieved using an associated string key.

(closes issue ASTERISK-22394)
Reported by: Rusty Newton
Review: https://reviewboard.asterisk.org/r/2843/
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Merged revisions 399990 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399991 65c4cc65-6c06-0410-ace0-fbb531ad65f3