json-parser: Fix segfault on malformed input

If the parser fails to parse the key in parse_pair, it will access a NULL
pointer. A simple way to trigger this is sending {foo} via QMP. This patch
turns the segfault into a syntax error reply.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>
(cherry picked from commit d758d90f)

json-parser.c

@@ -266,7 +266,7 @@ static int parse_pair(JSONParserContext *ctxt, QDict *dict, QList **tokens, va_l
 
     peek = qlist_peek(working);
     key = parse_value(ctxt, &working, ap);
-    if (qobject_type(key) != QTYPE_QSTRING) {
+    if (!key || qobject_type(key) != QTYPE_QSTRING) {
         parse_error(ctxt, peek, "key is not a string in object");
         goto out;
     }