i3bar: handle clicks with negative coordinates (Thanks Julian)

This can happen if you move your mouse pointer to the very left of the
screen and then click. For better usability, we handle this edge case
like a click on pixel 0.

i3bar/src/xcb.c

@@ -281,7 +281,7 @@ void handle_button(xcb_button_press_event_t *event) {
         return;
     }
 
-    int32_t x = event->event_x;
+    int32_t x = event->event_x >= 0 ? event->event_x : 0;
 
     DLOG("Got Button %d\n", event->detail);