target: add valid smp target check

Check that the target is valid before calling any target functions.

Change-Id: I538fccc79d5ec89976e14beab02cb20490b299bb
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/766
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

src/target/target.c

@@ -5131,8 +5131,10 @@ static int jim_target_smp(Jim_Interp *interp, int argc, Jim_Obj *const *argv)
 		target->head = head;
 		curr = curr->next;
 	}
-	if (target->rtos)
+
+	if (target && target->rtos)
 		retval = rtos_smp_init(head->target);
+
 	return retval;
 }