Merge bitcoin-core/secp256k1#996: Fix G.y parity in sage code

044d956 Fix G.y parity in sage code (Pieter Wuille)

Pull request description:

  I'm not sure if `EllipticCurve.lift_x` has well-defined Y coordinate or not, but at least my current version of Sage computes the wrong G. Fix this.

ACKs for top commit:
  real-or-random:
    ACK 044d956

Tree-SHA512: afb919af29027da2bb3c58628924f9740672d3c347ad39cc663c9c399b1aa8536256fd3fd4e1e54457e38344704d47f281d82488da413f4e6e67e191decc960f

sage/secp256k1_params.sage

@@ -9,6 +9,9 @@ C = EllipticCurve([F(0), F(7)])
 
 """Base point of secp256k1"""
 G = C.lift_x(0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798)
+if int(G[1]) & 1:
+    # G.y is even
+    G = -G
 
 """Prime order of secp256k1"""
 N = C.order()