json: Fix a memleak in parse_pair()

json: Fix a memleak in parse_pair()

In qobject_type(), NULL is returned when the 'QObject' returned from parse_value() is not of QString type,
and this 'QObject' memory will leaked.
So we need to first cache the 'QObject' returned from parse_value(), and finally
free 'QObject' memory at the end of the function.
Also, we add a testcast about invalid dict key.

The memleak stack is as follows:
Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0xfffe4b3c34fb in __interceptor_malloc (/lib64/libasan.so.4+0xd34fb)
    #1 0xfffe4ae48aa3 in g_malloc (/lib64/libglib-2.0.so.0+0x58aa3)
    #2 0xaaab3557d9f7 in qnum_from_int qemu/qobject/qnum.c:25
    #3 0xaaab35584d23 in parse_literal qemu/qobject/json-parser.c:511
    #4 0xaaab35584d23 in parse_value qemu/qobject/json-parser.c:554
    #5 0xaaab35583d77 in parse_pair qemu/qobject/json-parser.c:270
    #6 0xaaab355845db in parse_object qemu/qobject/json-parser.c:327
    #7 0xaaab355845db in parse_value qemu/qobject/json-parser.c:546
    #8 0xaaab35585b1b in json_parser_parse qemu/qobject/json-parser.c:580
    #9 0xaaab35583703 in json_message_process_token qemu/qobject/json-streamer.c:92
    #10 0xaaab355ddccf in json_lexer_feed_char qemu/qobject/json-lexer.c:313
    #11 0xaaab355de0eb in json_lexer_feed qemu/qobject/json-lexer.c:350
    #12 0xaaab354aff67 in tcp_chr_read qemu/chardev/char-socket.c:525
    #13 0xfffe4ae429db in g_main_context_dispatch (/lib64/libglib-2.0.so.0+0x529db)
    #14 0xfffe4ae42d8f  (/lib64/libglib-2.0.so.0+0x52d8f)
    #15 0xfffe4ae430df in g_main_loop_run (/lib64/libglib-2.0.so.0+0x530df)
    #16 0xaaab34d70bff in iothread_run qemu/iothread.c:82
    #17 0xaaab3559d71b in qemu_thread_start qemu/util/qemu-thread-posix.c:519

Fixes: 532fb5328473 ("qapi: Make more of qobject_to()")
Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201113145525.85151-1-alex.chen@huawei.com>
[Commit message tweaked]