introduce jinvalid value

:Release Notes:
Made a difference between jnull() and invalid value by introducing
jinvalid().

:Detailed Notes:
- added jis_valid to check answer for erroneous result
- extended jis_null to cover jinvalid() also
- jinvalid() returns JV_NULL value

:Testing Performed:
unit tests and performance test on desktop

:QA Notes:

:Issues Addressed:
[ANG-547] pbnjson: clash between error and null-value as result of jdom_parse

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: I85b57891976a4bd6a8ef4ed74fec99d8f0b816ec
Reviewed-on: https://g2g.palm.com/4538
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>