Fixed issue where DRBG_CTR fails if NO_DF is used - when entropy is called

Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NKurt Roeckx <kurt@roeckx.be>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6778)

crypto/rand/rand_lib.c

@@ -174,7 +174,7 @@ size_t rand_drbg_get_entropy(RAND_DRBG *drbg,
             if (RAND_DRBG_generate(drbg->parent,
                                    buffer, bytes_needed,
                                    prediction_resistance,
-                                   (unsigned char *)drbg, sizeof(*drbg)) != 0)
+                                   NULL, 0) != 0)
                 bytes = bytes_needed;
             rand_drbg_unlock(drbg->parent);