sd-bus: make sure dispatch_rqueue() initializes return parameter on all types of success

Let's make sure our own code follows coding style and initializes all
return values on all types of success (and leaves it uninitialized in
all types of failure).

src/libsystemd/sd-bus/sd-bus.c

@@ -1865,8 +1865,10 @@ static int dispatch_rqueue(sd_bus *bus, bool hint_priority, int64_t priority, sd
                 r = bus_read_message(bus, hint_priority, priority);
                 if (r < 0)
                         return r;
-                if (r == 0)
+                if (r == 0) {
+                        *m = NULL;
                         return ret;
+                }
 
                 ret = 1;
         }