Align BIO_get_fd with upstream.

OpenSSL's BIO_get_fd returns the fd or -1, not a boolean.

Change-Id: I12a3429c71bb9c9064f9f91329a88923025f1fb5
Reviewed-on: https://boringssl-review.googlesource.com/6080
Reviewed-by: Adam Langley <agl@google.com>