res_pjsip: Fix contact refleak on stateful responses.

When sending a stateful response, creation of the transaction can fail,
most commonly because we are trying to create a transaction from a
retransmitted request. When creation of the transaction fails, we end up
leaking a reference to a contact that was bumped when the response was
created.

This patch adds the missing deref and fixes the reference leak.

Change-Id: I2f97ad512aeb1b17e87ca29ae0abacb4d6395f07