Correct num expected inputs calc for multisig.

This commit corrects the number of expected inputs for a multi-sig script
to include the additional item that is popped from the stack due to the
OP_CHECKMULTISIG consensus bug (which is required and properly performed).

Note this issue did NOT affect the consensus critical code and hence would
not cause a chain fork.  It did however, cause standard p2sh multisig txns
to be rejected from the mempool as nonstandard.

The tx rejected as non-standard which prompted this was spotted by
@mbelshe on IRC.

ok @owainga