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>