input/MouseSettings: Fix return to NULL per clang

* This looks wrong, but is functionally equal.
* Supposed to return a MouseSettings*, but returned
  a B_OK (0) on a seemingly error result (NULL).

Change-Id: Id6c50369790b90e38fd6ec037c466c5fb314edf5
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3303
Reviewed-by: Adrien Destugues <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/servers/input/MouseSettings.cpp

@@ -427,7 +427,7 @@ MultipleMouseSettings::AddMouseSettings(BString mouse_name)
 			(mouse_name, settings));
 		return settings;
 	}
-	return B_OK;
+	return NULL;
 }