If the mousebutton is unknown when we go to grab, don't just print a

warning, but also skip the XGrabButton call. Noticed by code inspection
by okan@, but we agreed my fix was cleaner.

ok okan.

conf.c

@@ -528,6 +528,7 @@ conf_grab_mouse(struct client_ctx *cc)
 			break;
 		default:
 			warnx("strange button in mousebinding\n");
+			continue;
 		}
 		xu_btn_grab(cc->pwin, mb->modmask, button);
 	}