Fix a memleak in the FIPS provider

If the DRBG is used within the scope of the FIPS OSSL_provider_init
function then it attempts to register a thread callback via c_thread_start.
However the implementation of c_thread_start assumed that the provider's
provctx was already present. However because OSSL_provider_init is still
running it was actually NULL. This means the thread callback fail to work
correctly and a memory leak resulted.

Instead of having c_thread_start use the provctx as the callback argument
we change the definition of c_thread_start to have an explicit callback
argument to use.

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15278)
