Fix crash in string serialization if there is a error in jschema_init

[GF-2746] pbnjson::JGenerator::toString ignores schema

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

Change-Id: Ic7988640c8fd4a6574742e2e6e10d326cc9568c2