Avoid division by zero in hybrid point encoding

Avoid division by zero in hybrid point encoding

In hybrid and compressed point encodings, the form octet contains a bit
of information allowing to calculate y from x.  For a point on a binary
curve, this bit is zero if x is zero, otherwise it must match the
rightmost bit of of the field element y / x.  The existing code only
considers the second possibility. It could thus incorrecly fail with a
division by zero error as found by Guido Vranken's cryptofuzz.

This commit adds a few explanatory comments to oct2point. The only
actual code change is in the last hunk which adds a BN_is_zero(x)
check to avoid the division by zero.

Fixes #15021

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15112)