qobject: Let qobject_from_jsonf() fail instead of abort

qobject_from_jsonf() aborts on error, unlike qobject_from_jsonv(),
which returns null.  Since all remaining users of qobject_from_jsonf()
cope fine with null, change it to return null.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-30-armbru@redhat.com>

qobject/qjson.c

@@ -59,10 +59,6 @@ QObject *qobject_from_json(const char *string, Error **errp)
     return qobject_from_jsonv(string, NULL, errp);
 }
 
-/*
- * IMPORTANT: This function aborts on error, thus it must not
- * be used with untrusted arguments.
- */
 QObject *qobject_from_jsonf(const char *string, ...)
 {
     QObject *obj;
@@ -72,7 +68,6 @@ QObject *qobject_from_jsonf(const char *string, ...)
     obj = qobject_from_jsonv(string, &ap, &error_abort);
     va_end(ap);
 
-    assert(obj != NULL);
     return obj;
 }