rtp_engine.c: Prevent segfault in ast_rtp_codecs_payloads_unset()

rtp_engine.c: Prevent segfault in ast_rtp_codecs_payloads_unset()

There can be empty slots in payload_mapping_tx corresponding to
dynamic payload types that haven't been seen before so we now
check for NULL before attempting to use 'type' in the call to
ast_format_cmp.

Note: Currently only chan_sip calls ast_rtp_codecs_payloads_unset()

Resolves: #822