fix op_checksig with additional op_0 on stack

  found in mainnet tx a6ce7081addade7676cd2af75c4129eba6bf5e179a19c40c7d4cf6a5fe595954 output 0
add missing network values to old :testnet too
fix op_checksig with empty signature

  the previous fix was wrong. op_0 must not be dropped, but be used as signature and fail then because of the length check.
fix op_checkmultisig with empty signature

  OP_0 as signature should fail before parse_sig
remove triplicate call to #get_header when computing blockchain locator (thanks lian!)