pjproject_bundled: Prevent crash on bad outgoing header

We still need to figure out how a bad header is getting into the
outgoing message but this patch to pjproject prevents attempting
to print that header and causing a crash.

For several users, this crash happens when sending 183 progress
messages.

ASTERISK-26832
Reported by: Ross Beer, Jan Rozhon

Change-Id: Ie5c5a921c890c843587763e7f33f987dfe66bd16