Guard against zero-sized effect destination boxes

Co-authored-by: dy-tea <165158232+dy-tea@users.noreply.github.com>

Author: Copilot

src/effects.c

@@ -422,14 +422,19 @@ static bool compute_src_box(output_t *output, const struct wlr_box *r, struct wl
 	if (sw <= 0.0f || sh <= 0.0f)
 		return false;
 
+	int dw = (int)sw;
+	int dh = (int)sh;
+	if (dw <= 0 || dh <= 0)
+		return false;
+
 	*src_out = (struct wlr_fbox){
 		.x = sx,
 		.y = sy,
 		.width = sw,
 		.height = sh
 	};
-	*dw_out = (int)sw;
-	*dh_out = (int)sh;
+	*dw_out = dw;
+	*dh_out = dh;
 	return true;
 }