Fix JSON parser handling for certain objects (#7162)

This PR also adds some comments and simplifies
the code. Primary handling is done to ensure
that we make sure to honor cached buffer.

Added unit tests as well

Fixes #7141

vendor/github.com/bcicen/jstream/scratch.go

@@ -0,0 +1,44 @@
+package jstream
+
+import (
+	"unicode/utf8"
+)
+
+type scratch struct {
+	data []byte
+	fill int
+}
+
+// reset scratch buffer
+func (s *scratch) reset() { s.fill = 0 }
+
+// bytes returns the written contents of scratch buffer
+func (s *scratch) bytes() []byte { return s.data[0:s.fill] }
+
+// grow scratch buffer
+func (s *scratch) grow() {
+	ndata := make([]byte, cap(s.data)*2)
+	copy(ndata, s.data[:])
+	s.data = ndata
+}
+
+// append single byte to scratch buffer
+func (s *scratch) add(c byte) {
+	if s.fill+1 >= cap(s.data) {
+		s.grow()
+	}
+
+	s.data[s.fill] = c
+	s.fill++
+}
+
+// append encoded rune to scratch buffer
+func (s *scratch) addRune(r rune) int {
+	if s.fill+utf8.UTFMax >= cap(s.data) {
+		s.grow()
+	}
+
+	n := utf8.EncodeRune(s.data[s.fill:], r)
+	s.fill += n
+	return n
+}