res_pjsip_session: Fix memory leak of direct media format capabilities

The direct media format capabilities are always allocated in
ast_sip_session_alloc and were not freed in the session destructor. Whoops.

(This being the third whoops caught by Scott and Nitesh's valgrind work for
the Asterisk Test Suite. Nifty!)
........

Merged revisions 402968 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@402969 f38db490-d61c-443f-a65b-d21fe96a405b