Add rejection of non-null CHECKMULTISIG dummy values

This is a source of transaction mutability as the dummy value was
previously not checked and could be modified to something other than the
usual OP_0 value.

src/script.h

@@ -42,6 +42,7 @@ enum
     SCRIPT_VERIFY_STRICTENC = (1U << 1), // enforce strict conformance to DER and SEC2 for signatures and pubkeys
     SCRIPT_VERIFY_EVEN_S    = (1U << 2), // enforce even S values in signatures (depends on STRICTENC)
     SCRIPT_VERIFY_NOCACHE   = (1U << 3), // do not store results in signature cache (but do query it)
+    SCRIPT_VERIFY_NULLDUMMY = (1U << 4), // verify dummy stack item consumed by CHECKMULTISIG is of zero-length
 };
 
 // Mandatory script verification flags that all new blocks must comply with for
@@ -54,7 +55,8 @@ static const unsigned int MANDATORY_SCRIPT_VERIFY_FLAGS = SCRIPT_VERIFY_P2SH;
 // with. However scripts violating these flags may still be present in valid
 // blocks and we must accept those blocks.
 static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY_FLAGS |
-                                                         SCRIPT_VERIFY_STRICTENC;
+                                                         SCRIPT_VERIFY_STRICTENC |
+                                                         SCRIPT_VERIFY_NULLDUMMY;
 
 enum txnouttype
 {