Fix bug in entropy gathering.

This only impacts FIPS mode or someone who has enabled the FIPS 140.2
4.9.2 Conditional Tests.  i.e. nobody currently.

Fix a significant issue in the entropy gathering for the continuous RNG
testing.  The impact is using an uninitialised buffer instead of the gathered
entropy.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/8789)