RAND_load_file(): return error if reseeding failed

The failure of RAND_load_file was only noticed because of the
heap corruption which was reported in #7499 and fixed in commit
5b4cb385

. To prevent this in the future, RAND_load_file()
now explicitly checks RAND_status() and reports an error if it
fails.

Related-to: #7449

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/7456)