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 fail with a division by
zero error as found by Guido Vranken's cryptofuzz.

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

ok jsing