OPENSSL_LH_flush(): assign NULL after freeing

OPENSSL_LH_flush() frees the linked lists for each slot, but didn't
set the list head to NULL after doing so, with the result that an
operation that affects these lists is likely to cause a crash.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8781)