native: use proper stack for result

When native method calls other contract result should be put
on the stack of current context. With oracles this problem wasn't
noticed because of void return type.

pkg/core/interop_system_test.go

@@ -454,7 +454,7 @@ func getTestContractState() (*state.Contract, *state.Contract) {
 		{
 			Name:       "justReturn",
 			Offset:     justRetOff,
-			ReturnType: smartcontract.IntegerType,
+			ReturnType: smartcontract.VoidType,
 		},
 		{
 			Name:       manifest.MethodVerify,