json-parser: use length parameter when validating utf-8
json-parser: use length parameter when validating utf-8

https://bugzilla.gnome.org/show_bug.cgi?id=727755

ebassi/json-glib,ebassi/json-glib,frida/json-glib,Distrotech/json-glib,frida/json-glib,GNOME/json-glib,Distrotech/json-glib,ebassi/json-glib,GNOME/json-glib,Distrotech/json-glib