fix: invalid infinity point decoding throws error

[gbotrel]

Thanks to @asanso and @kevaundray for reporting;

IETF spec we followed states:

The serialization of a point at infinity comprises a string of zero bytes, except that the metadata bits may be nonzero.¶

Previously, we did check the metadata bits and if they were signaling a point at infinity, discarded the rest of the buffer. We now add an additional check to ensure all the bytes are at 0.

[yelhousni]

LGTM