image: remove unrequired checks before g_set_error()
image: remove unrequired checks before g_set_error()

g_set_error() already checks whether **error == NULL, the extra check is
just pointless branching, which is somewhat ironic when someone has
gone out of their way to provide branch prediction hints.

pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx