Fix a buffer overflow in drbg_ctr_generate

This can happen if the 32-bit counter overflows
and the last block is not a multiple of 16 bytes.

Fixes #12012

[extended tests]

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Patrick Steuer <patrick.steuer@de.ibm.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/12016)