json-parser: Fix potential NULL pointer segfault

Report from smatch:
json-parser.c:474 parse_object(62) error: potential null derefence 'dict'.
json-parser.c:553 parse_array(75) error: potential null derefence 'list'.

Label 'out' in json-parser.c can be called with list == NULL
which is passed to QDECREF.

Modify QDECREF to handle a NULL argument (inline function qobject_decref
already handles them, too).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
