Chaos: make hangup NULL tolerant

In chan_pjsip_new, if allocation of the pvt
structure fails, ast_hangup is called.  But
it was written to assume pvt was valid, and
this change corrects that.

ASTERISK-25323
Reported by: Scott Griepentrog

Change-Id: I5f47860fe9cee4cd56abd3f79b108678ab72cc87