Avoid an infinite loop in BN_GF2m_mod_inv

Avoid an infinite loop in BN_GF2m_mod_inv

If p is set to 1 when calling BN_GF2m_mod_inv then an infinite loop will
result. Calling this function set 1 when applications call this directly
is a non-sensical value - so this would be considered a bug in the caller.

It does not seem possible to cause OpenSSL internal callers of
BN_GF2m_mod_inv to call it with a value of 1.

So, for the above reasons, this is not considered a security issue.
Reported by Bing Shi.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/22960)