Fix an endless loop in BN_generate_prime_ex

Happens when trying to generate 4 or 5 bit safe primes.

[extended tests]

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9311)

(cherry picked from commit 291f616ced45c924d639d97fc9ca2cbeaad096cf)