script: Change SignatureHash input index check to an assert.

In the SignatureHash function, the input index must refer to a valid
index. This is not enforced equally in the segwit/non-segwit branches
and should be an assertion rather than returning a error hash.

src/script/interpreter.cpp

@@ -1175,6 +1175,8 @@ PrecomputedTransactionData::PrecomputedTransactionData(const CTransaction& txTo)
 
 uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType, const CAmount& amount, SigVersion sigversion, const PrecomputedTransactionData* cache)
 {
+    assert(nIn < txTo.vin.size());
+
     if (sigversion == SIGVERSION_WITNESS_V0) {
         uint256 hashPrevouts;
         uint256 hashSequence;
@@ -1221,10 +1223,6 @@ uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsig
     }
 
     static const uint256 one(uint256S("0000000000000000000000000000000000000000000000000000000000000001"));
-    if (nIn >= txTo.vin.size()) {
-        //  nIn out of range
-        return one;
-    }
 
     // Check for invalid use of SIGHASH_SINGLE
     if ((nHashType & 0x1f) == SIGHASH_SINGLE) {