fix OP_CHECKMULTISIG p2sh with empty signature

as seen in mainnet tx: 136becd0892fa38c5aca8104db8b90b3a0e6b40912b7d1462aed583c067054cd

Not sure if this is the best way to fix it, ideally the case should be caught more generically in #parse_sig.
But it can't simply fail the script, it needs to skip the operation it was trying to run on the empty sig, and leave a 0 on the stack. No idea how to do that from inside #parse_sig...