res_pjsip_sdp_rtp fix leaking astobj2 ast_format

res_pjsip_sdp_rtp fix leaking astobj2 ast_format

PR #700 added a preferred_format for the struct ast_rtp_codecs,
but when set the preferred_format it leaks an astobj2 ast_format.
In the next code
ast_rtp_codecs_set_preferred_format(&codecs, ast_format_cap_get_format(joint, 0));
both functions ast_rtp_codecs_set_preferred_format
and ast_format_cap_get_format increases the ao2 reference count.

Fixes: #856