Fix spurious bn_test failures.

BN_rand generates a single-word zero BIGNUM with quite a large
probability.

A zero BIGNUM in turn will end up having a NULL |d|-buffer, which we
shouldn't dereference without checking.

(Imported from upstream's 9c989aaa749d88b63bef5d5beeb3046eae62d836.)

Change-Id: Ic4d113e4fcf4ea4c0a4e905a1c4ba3fb758d9fc6
Reviewed-on: https://boringssl-review.googlesource.com/5782
Reviewed-by: Adam Langley <agl@google.com>