cursor: ensure mapping box is always initialized

Commit 498f30aad1 changed the logic of
get_mapping() in types/wlr_cursor.c to use updated version of
wlr_output_layout_get_box(). However, the case where c_device isn't NULL
but doesn't have output or geometry mappings wasn't handled properly,
resulting in leaving the output value uninitialized. This commit fixes
`c_device != NULL` branch by returning from the function only when a
mapping is found.

Fixes https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3369

types/wlr_cursor.c

@@ -248,11 +248,12 @@ static void get_mapping(struct wlr_cursor *cur,
 	if (c_device) {
 		if (!wlr_box_empty(&c_device->mapped_box)) {
 			*box = c_device->mapped_box;
+			return;
 		} else if (c_device->mapped_output) {
 			wlr_output_layout_get_box(cur->state->layout,
 				c_device->mapped_output, box);
+			return;
 		}
-		return;
 	}
 
 	if (!wlr_box_empty(&cur->state->mapped_box)) {