Don't rely on libcrypto returning static buffers

Don't rely on libcrypto returning static buffers

ldns is one of very few applications relying on being able to pass
NULL as last argument to the one-step hashing functions. BoringSSL
has removed this functionality in 2017 [1] and LibreSSL 4.0 will do
the same. Applications can pass in a correctly-sized buffer on the
stack.

[1]: https://boringssl-review.googlesource.com/14528