RAND_get_rand_method() can return a NULL method pointer in the case of a
malloc failure, so don't dereference it without a check.

Reported-by: Zu-Ming Jiang (detected by FIFUZZ)

Fixes #10480
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/10490)