Fix libinput_device_group_find_group() to return NULL on failure

struct list isn't a null-terminated list, list_for_each() causes 'g' to be set
to the list head at the end of the loop. Returning that as group caused random
memory to be overwritten.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

src/libinput.c

@@ -2011,7 +2011,7 @@ libinput_device_group_find_group(struct libinput *libinput,
 		}
 	}
 
-	return g;
+	return NULL;
 }
 
 void