res_pjsip_sdp_rtp,sorcery: Fix invalid access and memory leak respectively.

Valgrind found a memory leak and invalid access.

* Fix invalid access by sscanf() being fed a non-nul terminated string of
digits in res/res_pjsip_sdp_rtp.c:get_codecs().

* Fix memory leak in main/sorcery.c:sorcery_object_field_destructor().

* Fix potential NULL pointer dereference in
main/xmldoc.c:xmldoc_get_syntax_config_option().

Review: https://reviewboard.asterisk.org/r/4513/
........

Merged revisions 433199 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433200 65c4cc65-6c06-0410-ace0-fbb531ad65f3