linux-dmabuf: Fix crash with no valid modifiers

We shouldn't free &modifier_invalid because it wasn't allocated
with malloc()

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

libweston/linux-dmabuf.c

@@ -509,7 +509,8 @@ bind_linux_dmabuf(struct wl_client *client,
 							  modifier_hi,
 							  modifier_lo);
 		}
-		free(modifiers);
+		if (modifiers != &modifier_invalid)
+			free(modifiers);
 	}
 	free(formats);
 }