Avoid multiple locks in FIPS mode.

PR: 3176.

In FIPS mode ssleay_rand_bytes is only used for PRNG seeding and is
performed in either a single threaded context (when the PRNG is first
initialised) or under a lock (reseeding). To avoid multiple locks disable
use of CRYPTO_LOCK_RAND in FIPS mode in ssleay_rand_bytes.