RAND_load_file(): avoid adding small chunks to RAND_add()

Increase the load buffer size such that it exceeds the chunk
size by a comfortable amount. This is done to avoid calling
RAND_add() with a small final chunk. Instead, such a small
final chunk will be added together with the previous chunk
(unless it's the only one).

Related-to: #7449

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