res_pjsip: Only check transaction on transaction state events.

The send request callback function currently assumes that it
will only ever be called on transaction state changes. This is
not always true. If our own timer callback occurs we will call
the callback with a timer event instead of a transaction state
change event. In this case the transaction on the event is
invalid and accessing it will result in a crash.

ASTERISK-26049 #close

Change-Id: I623211c8533eb73056b0250b4580b49ad4174dfc