Only allocate array if required (buger#173)

My benchmarking showed that this array was actually leaking to the heap. Narrowing its scope in the hope of it not escaping, but also limiting its allocation only to cases where it is actually required.

Author: richardartoul

parser.go

@@ -980,7 +980,6 @@ func ArrayEach(data []byte, cb func(value []byte, dataType ValueType, offset int
 
 // ObjectEach iterates over the key-value pairs of a JSON object, invoking a given callback for each such entry
 func ObjectEach(data []byte, callback func(key []byte, value []byte, dataType ValueType, offset int) error, keys ...string) (err error) {
-	var stackbuf [unescapeStackBufSize]byte // stack-allocated array for allocation-free unescaping of small strings
 	offset := 0
 
 	// Descend to the desired key, if requested
@@ -1034,6 +1033,7 @@ func ObjectEach(data []byte, callback func(key []byte, value []byte, dataType Va
 
 		// Unescape the string if needed
 		if keyEscaped {
+			var stackbuf [unescapeStackBufSize]byte // stack-allocated array for allocation-free unescaping of small strings
 			if keyUnescaped, err := Unescape(key, stackbuf[:]); err != nil {
 				return MalformedStringEscapeError
 			} else {