'modular inverse does not exist' when doubling a point on the x axis

[lc6chang]

When doubling a point on x axis (i.e. y=0), the expected result is the Point at infinity but now it raises 'modular inverse does not exist' exception.

>>> from ecc.curve import Curve25519
>>> from ecc.curve import Point
>>> P = Point(x=0, y=0, curve=Curve25519)
>>> P + P
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/chang/ecc-pycrypto/ecc/curve.py", line 40, in __add__
    return self.curve.add_point(self, other)
  File "/Users/chang/ecc-pycrypto/ecc/curve.py", line 104, in add_point
    return self._double_point(P)
  File "/Users/chang/ecc-pycrypto/ecc/curve.py", line 255, in _double_point
    s = up * modinv(down, self.p)
  File "/Users/chang/ecc-pycrypto/ecc/math_utils/mod_inverse.py", line 16, in modinv
    raise Exception("modular inverse does not exist")
Exception: modular inverse does not exist