geom: Assert that BIO_SPEEDUP BIOs have bio_data set to NULL

Like BIO_FLUSH, there is no reason for consumers to pass a BIO_SPEEDUP
request with non-NULL bio_data, so assert this.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 38da0c96dc1b4698931f65d8d58e864dfc3a600e)