mp.c: Release the `GFN' object through channels on error.

mp.c: Release the `GFN' object through channels on error.

If the given element turns out not to actually generate a normal basis
then we have to give up constructing the `GFN' object and raise an
exception.  In turns out that debug versions of Python get really
unhappy if you try to free objects which still have nonzero reference
counts, so:

  * use `Py_DECREF' to free the object on error; and

  * mark the object (by leaving `p' null) so that we don't actually
    free the conversion matrices if they're weren't set up.