Fix off-by-one error in BN_bn2hex

A BIGNUM can have the value of -0. The function BN_bn2hex fails to account
for this and can allocate a buffer one byte too short in the event of -0
being used, leading to a one byte buffer overrun. All usage within the
OpenSSL library is considered safe. Any security risk is considered
negligible.

With thanks to Mateusz Kocielski (LogicalTrust), Marek Kroemeke and
Filip Palian for discovering and reporting this issue.

Reviewed-by: Tim Hudson <tjh@openssl.org>