crypto: drbg - fix failure of generating multiple of 2**16 bytes

The function drbg_generate_long slices the request into 2**16 byte
or smaller chunks. However, the loop, however invokes the random number
generation function with zero bytes when the request size is a multiple
of 2**16 bytes. The fix prevents zero bytes requests.
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/drbg.c

@@ -1500,7 +1500,7 @@ static int drbg_generate_long(struct drbg_state *drbg,
 		if (0 >= tmplen)
 			return tmplen;
 		len += tmplen;
-	} while (slice > 0);
+	} while (slice > 0 && (len < buflen));
 	return len;
 }