node.js uses a memory BIO in the wrong mode which, for now, we work
around. It also passes in NULL (rather than empty) strings and a
non-NULL out-arg for |d2i_PKCS12_bio|.

Change-Id: Ib565b4a202775bb32fdcb76db8a4e8c54268c052
Reviewed-on: https://boringssl-review.googlesource.com/7012
Reviewed-by: Adam Langley <agl@google.com>