Remove nil == nil if-statement

Found using the nilness analysis:
```
$ go install golang.org/x/tools/go/analysis/passes/nilness/cmd/nilness
$ go vet -vettool=$(which nilness)
\# github.com/json-iterator/go
./iter_object.go:241:10: tautological condition: nil == nil
```

Author: Robert Welin

iter_object.go

@@ -238,10 +238,6 @@ func (iter *Iterator) readObjectStart() bool {
 func (iter *Iterator) readObjectFieldAsBytes() (ret []byte) {
 	str := iter.ReadStringAsSlice()
 	if iter.skipWhitespacesWithoutLoadMore() {
-		if ret == nil {
-			ret = make([]byte, len(str))
-			copy(ret, str)
-		}
 		if !iter.loadMore() {
 			return
 		}