res/res_pjsip_session.c: Check that media type matches in

function ast_sip_session_media_state_add.

Check ast_media_type matches when a ast_sip_session_media is found
otherwise when transitioning from say image to audio, the wrong
session is returned in the first if statement.

ASTERISK-29220 #close

Change-Id: I6f6efa9b821ebe8881bb4c8c957f8802ddcb4b5d