res_pjsip_session: Correct inverted test in session_outgoing_nat_hook

There was a typo introduced in commit 776ffd77 which was preventing
the transport's external media address from being used.

ASTERISK-27024 #close
Reported-by: Christopher van de Sande
patches:
	patch.diff submitted by Florian Floimair (license 6892)

Change-Id: I7ec617171eaa2d86d2680b00cf37d5088adafc27