Merge pull request #300 from dragos-oancea/sofia-nua_stack-nullptr

[sofia-sip] scan-build: Access to field 'nh_destroyed' results in a dereference of a null pointer (loaded from variable 'nh')

libs/sofia-sip/libsofia-sip-ua/nua/nua_stack.c

@@ -657,7 +657,7 @@ void nua_stack_signal(nua_t *nua, su_msg_r msg, nua_ee_data_t *ee)
     nua_stack_respond(nua, nh, e->e_status, e->e_phrase, tags);
     break;
   case nua_r_destroy:
-	  if (!nh->nh_destroyed) {
+	  if (nh && !nh->nh_destroyed) {
 		  nua_stack_destroy_handle(nua, nh, tags);
 		  su_msg_destroy(nua->nua_signal);
 	  }