Fix a rare panic importing discovered multisignature scripts
Fix a rare panic importing discovered multisignature scripts

A rare panic could occur importing multisignature scripts found in the
blockchain that were unconfirmed. This avoids the nil dereference that
causes the panic.

jrick/dcrwallet,jrick/dcrwallet,jrick/btcwallet,jrick/btcwallet,decred/dcrwallet,decred/dcrwallet