md_rand.c: don't stop polling until properly initialized

Previously, the RNG sets `initialized=1` after the first call to
RAND_poll(), although its criterion for being initialized actually
is whether condition `entropy >= ENTROPY_NEEDED` is true.

This commit now assigns `initialized=(entropy >= ENTROPY_NEEDED)`,
which has the effect that on the next call, RAND_poll() will be
called again, if it previously failed to obtain enough entropy.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7438)