rtp_engine.c: Get current or create a needed rx payload type mapping.

* Make ast_rtp_codecs_payload_code() get the current mapping or create a
rx payload type mapping.

ASTERISK-25166
Reported by: Kevin Harwell

ASTERISK-17410
Reported by: Boris Fox

Change-Id: Ia4b2d45877a8f004f6ce3840e3d8afe533384e56