Add another assertion about the witness cache.
Add another assertion about the witness cache.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash,bitcoinsSG/zcash