Fix: uninstantiation breaks the RAND_DRBG callback mechanism

Fix: uninstantiation breaks the RAND_DRBG callback mechanism

The RAND_DRBG callbacks are wrappers around the EVP_RAND callbacks.
During uninstantiation, the EVP_RAND callbacks got lost while the
RAND_DRBG callbacks remained, because RAND_DRBG_uninstantiate()
calls RAND_DRBG_set(), which recreates the EVP_RAND object.
This was causing drbgtest failures.

This commit fixes the problem by adding code to RAND_DRBG_set() for
saving and restoring the EVP_RAND callbacks.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11195)