[ClutterTexture] Check before unrefing the fbo_handle

When replacing the fbo_handle with a new handle it first unrefs the
old handle. This was previously a call to cogl_offscreen_unref which
silently ignored attempts to unref COGL_INVALID_HANDLE. However the
new cogl_handle_unref does check for this so we should make sure the
handle is valid to avoid the warning.

clutter/clutter-texture.c

@@ -2303,6 +2303,7 @@ on_fbo_source_size_change (GObject          *object,
       CoglHandle tex;
 
       /* tear down the FBO */
+      if (priv->fbo_handle != COGL_INVALID_HANDLE)
         cogl_handle_unref (priv->fbo_handle);
 
       texture_free_gl_resources (texture);