DRBG: fix coverity issues

- drbg_lib.c: Silence coverity warning: the comment preceding the
  RAND_DRBG_instantiate() call explicitely states that the error
  is ignored and explains the reason why.

- drbgtest: Add checks for the return values of RAND_bytes() and
  RAND_priv_bytes() to run_multi_thread_test().

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5976)