Fix out-of-bounds read in ctr_XOR

Looking at
http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-90Ar1.pdf


we see that in the CTR_DRBG_Update() algorithm (internal page number 51),
the provided input data is (after truncation to seedlen) xor-d with the
key and V vector (of length keylen and blocklen respectively).  The comment
in ctr_XOR notes that xor-ing with 0 is the identity function, so we can
just ignore the case when the provided input is shorter than seedlen.

The code in ctr_XOR() then proceeds to xor the key with the input, up
to the amount of input present, and computes the remaining input that
could be used to xor with the V vector, before accessing a full 16-byte
stretch of the input vector and ignoring the calculated length.  The correct
behavior is to respect the supplied input length and only xor the
indicated number of bytes.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3971)