tests/qtest: fix memleak in npcm7xx_watchdog_timer-test

tests/qtest: fix memleak in npcm7xx_watchdog_timer-test

Properly free resp for get_watchdog_action() to avoid memory leak.
ASAN shows memory leak stack:

Indirect leak of 12360 byte(s) in 3 object(s) allocated from:
    #0 0x7f41ab6cbd4e in __interceptor_calloc (/lib64/libasan.so.5+0x112d4e)
    #1 0x7f41ab4eaa50 in g_malloc0 (/lib64/libglib-2.0.so.0+0x55a50)
    #2 0x556487d5374b in qdict_new ../qobject/qdict.c:29
    #3 0x556487d65e1a in parse_object ../qobject/json-parser.c:318
    #4 0x556487d65cb6 in parse_pair ../qobject/json-parser.c:287
    #5 0x556487d65ebd in parse_object ../qobject/json-parser.c:343
    #6 0x556487d661d5 in json_parser_parse ../qobject/json-parser.c:580
    #7 0x556487d513df in json_message_process_token ../qobject/json-streamer.c:92
    #8 0x556487d63919 in json_lexer_feed_char ../qobject/json-lexer.c:313
    #9 0x556487d63d75 in json_lexer_feed ../qobject/json-lexer.c:350
    #10 0x556487d28b2a in qmp_fd_receive ../tests/qtest/libqtest.c:613
    #11 0x556487d2a16f in qtest_qmp_eventwait_ref ../tests/qtest/libqtest.c:827
    #12 0x556487d248e2 in get_watchdog_action ../tests/qtest/npcm7xx_watchdog_timer-test.c:94
    #13 0x556487d25765 in test_enabling_flags ../tests/qtest/npcm7xx_watchdog_timer-test.c:243

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Message-Id: <20201118115646.2461726-3-kuhn.chenqun@huawei.com>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>