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

pkg/s3select/json/data/12.json

@@ -0,0 +1,5 @@
+{
+    "a": 1
+}{
+    "b": 2
+}