Fix TypeError when passing INFINITY to VerifyingKey.from_public_point()

[gaoflow]

Problem

Passing ellipticcurve.INFINITY to VerifyingKey.from_public_point() raises a TypeError instead of the documented MalformedPointError:

>>> from ecdsa.keys import VerifyingKey
>>> from ecdsa.ellipticcurve import INFINITY
>>> VerifyingKey.from_public_point(INFINITY)
TypeError: '<=' not supported between instances of 'int' and 'NoneType'

Reported in #341.

Root cause

INFINITY is Point(None, None, None), so INFINITY.x() returns None. Public_key.__init__ evaluated 0 <= point.x() < p before checking for the point at infinity, producing a TypeError. Additionally, PointJacobi.from_affine(INFINITY) wraps INFINITY into a PointJacobi with x=None, so the TypeError happened before the except ecdsa.InvalidPointError handler in from_public_point could convert it to MalformedPointError.

Fix

Two guard lines — one in each layer:

1. ecdsa.py – Public_key.__init__: check point == ellipticcurve.INFINITY and raise InvalidPointError before any coordinate access.
2. keys.py – VerifyingKey.from_public_point: check point == ellipticcurve.INFINITY and raise MalformedPointError before the from_affine conversion.

A regression test is included.

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This pull request was prepared with the assistance of AI, under my direction and review.