res_pjsip_logger: use the correct pointer when logging tx_messages to pcap

When writing tx messages to pcap files, Asterisk is using the wrong
pointer resulting in lots of wasted space. This patch fixes it to use
the correct pointer.

ASTERISK-28932 #close

Change-Id: I5b8253dd59a083a2ca2c81f232f1d14d33c6fd23