vm: introduce TryBool() for Element and use it in VerifyWitnesses

Script can return non-bool results that can still be converted to bool
according to the usual VM rules. Unfortunately Bool() panics if this
conversion fails which is OK for things done in vm.execute(), but certainly
not for VerifyWitnesses(), thus there is a need for TryBool() that will just
return an error in this case.

pkg/core/blockchain.go

@@ -1045,14 +1045,17 @@ func (bc *Blockchain) VerifyWitnesses(t *transaction.Transaction) error {
 		if vm.HasFailed() {
 			return errors.Errorf("vm failed to execute the script")
 		}
-		res := vm.PopResult()
-		switch res.(type) {
-		case bool:
-			if !(res.(bool)) {
+		resEl := vm.Estack().Pop()
+		if resEl != nil {
+			res, err := resEl.TryBool()
+			if err != nil {
+				return err
+			}
+			if !res {
 				return errors.Errorf("signature check failed")
 			}
-		default:
-			return errors.Errorf("vm returned non-boolean result")
+		} else {
+			return errors.Errorf("no result returned from the script")
 		}
 	}
 

pkg/vm/stack.go

@@ -80,30 +80,40 @@ func (e *Element) BigInt() *big.Int {
 	}
 }
 
-// Bool attempts to get the underlying value of the element as a boolean.
-// Will panic if the assertion failed which will be caught by the VM.
-func (e *Element) Bool() bool {
+// TryBool attempts to get the underlying value of the element as a boolean.
+// Returns error if can't convert value to boolean type.
+func (e *Element) TryBool() (bool, error) {
 	switch t := e.value.(type) {
 	case *BigIntegerItem:
-		return t.value.Int64() != 0
+		return t.value.Int64() != 0, nil
 	case *BoolItem:
-		return t.value
+		return t.value, nil
 	case *ArrayItem, *StructItem:
-		return true
+		return true, nil
 	case *ByteArrayItem:
 		for _, b := range t.value {
 			if b != 0 {
-				return true
+				return true, nil
 			}
 		}
-		return false
+		return false, nil
 	case *InteropItem:
-		return t.value != nil
+		return t.value != nil, nil
 	default:
-		panic("can't convert to bool: " + t.String())
+		return false, fmt.Errorf("can't convert to bool: " + t.String())
 	}
 }
 
+// Bool attempts to get the underlying value of the element as a boolean.
+// Will panic if the assertion failed which will be caught by the VM.
+func (e *Element) Bool() bool {
+	val, err := e.TryBool()
+	if err != nil {
+		panic(err)
+	}
+	return val
+}
+
 // Bytes attempts to get the underlying value of the element as a byte array.
 // Will panic if the assertion failed which will be caught by the VM.
 func (e *Element) Bytes() []byte {