Modify the returned value from "EncodePoint" and "DecodePoint"
Modify the returned value from "EncodePoint" and "DecodePoint"

As infinity point is a particular point in ECC, we decide that it
doesn't return an err when it is be encoded or decoded.
1.In the function of "EncodePoint", when X or Y of the PubKey is
infinity, return a byte of 0x00.
2.In the function of "DecodePoint", when the encodeData's first
byte is 0x00, return an infinity point whose X and Y are nil.

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