Add null check for m_resolver in JParser::resolve

[GF-5573] libpbnjson crashes if try to parse with schema containing
external refs and JDomParser is constructed with NULL JResolver

Open-webOS-DCO-1.0-Signed-off-by: Janne Hurskainen <janne.hurskainen@lge.com>

Change-Id: Id46111adf9ff6d9e74131ddc71e44b3f092dd0ca
Fix test_sax unit test

[GF-5205] libpbnjson test_sax unit test crashes after adding schema
validation

Open-webOS-DCO-1.0-Signed-off-by: Janne Hurskainen <janne.hurskainen@lge.com>

Change-Id: Id6ba7f04686ae16f32c6653f23e36cf72cadfc08