Replace uint256(1) with static constant

SignatureHash and its test function SignatureHashOld
return uint256(1) as a special error signaling value.
Return a local static constant with the same value instead.

src/script/interpreter.cpp

@@ -1030,16 +1030,17 @@ public:
 
 uint256 SignatureHash(const CScript& scriptCode, const CTransaction& txTo, unsigned int nIn, int nHashType)
 {
+    static const uint256 one("0000000000000000000000000000000000000000000000000000000000000001");
     if (nIn >= txTo.vin.size()) {
         //  nIn out of range
-        return 1;
+        return one;
     }
 
     // Check for invalid use of SIGHASH_SINGLE
     if ((nHashType & 0x1f) == SIGHASH_SINGLE) {
         if (nIn >= txTo.vout.size()) {
             //  nOut out of range
-            return 1;
+            return one;
         }
     }