bn: fix BN_DEBUG + BN_DEBUG_RAND support

Couple of updates to make this code work properly again;
* use OPENSSL_assert() instead of assert() (and #include <assert.h>)
* the circular-dependency-avoidance uses RAND_bytes() (not pseudo)

Change-Id: Iefb5a9dd73f71fd81c1268495c54a64378955354
Signed-off-by: Geoff Thorpe <geoff@openssl.org>

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1672)