AST-2018-005: Add a check for NULL tdata in ast_sip_failover_request

It was discovered that there are some corner cases where a pjsip tsx
might have no last_tx so calling ast_sip_failover_request with
a NULL last_tx as its tdata would cause a crash.

ASTERISK-27618
Reported By:  Sandro Gauci

Change-Id: Ic2b63f6d4ae617c4c19dcdec2a7a6156b54fd15b