cogl-auto-texture: Avoid a double-free crash

If texture allocation fails (e.g. on an old GPU with size limit 2048)
then `cogl_texture_new_with_size` was trying to use the same CoglError
twice. The second time was after it had already been freed.

Bug reported and fix provided by Gert van de Kraats.

https://launchpad.net/bugs/1790525

cogl/cogl/deprecated/cogl-auto-texture.c

@@ -94,6 +94,7 @@ cogl_texture_new_with_size (unsigned int width,
       if (!cogl_texture_allocate (tex, &skip_error))
         {
           cogl_error_free (skip_error);
+          skip_error = NULL;
           cogl_object_unref (tex);
           tex = NULL;
         }