buffer.c, ec.c: Fix required size for EC `buffer' encoding.

buffer.c, ec.c: Fix required size for EC `buffer' encoding.

The problem is zero coordinates: the point at infinity is encoded as a
zero length word, so zero coordinates must be encoded as a single zero
byte, preceded by a length word of 1 -- which overruns the output buffer
provided, unless we take special care, which we haven't.