prevent generator methods from returning NULL without setting an exception: method call protocol differs from __next__() protocol

mofe GIVEREF() before PyTuple_SET_ITEM() call because that's when we still own the reference (pypy needs it that way)

fix stupid C-API misuse: accidentally passed a NULL pointer to PyObject_GetItem() instead of a 0 integer index to PySequence_GetItem()

PyPy: Improve negative overflow error messages in int type conversion