qtest/npcm7xx_pwm-test: Fix memleak in pwm_qom_get

qtest/npcm7xx_pwm-test: Fix memleak in pwm_qom_get

The pwm_qom_get function didn't free "response", which caused an indirect
memory leak. So use qobject_unref() to fix it.

ASAN shows memory leak stack:

Indirect leak of 74160000 byte(s) in 18000 object(s) allocated from:
    #0 0x7f96e2f79d4e in __interceptor_calloc (/lib64/libasan.so.5+0x112d4e)
    #1 0x7f96e2d98a50 in g_malloc0 (/lib64/libglib-2.0.so.0+0x55a50)
    #2 0x556313112180 in qdict_new ../qobject/qdict.c:30
    #3 0x556313115bca in parse_object ../qobject/json-parser.c:318
    #4 0x556313117810 in parse_value ../qobject/json-parser.c:546
    #5 0x556313117bda in json_parser_parse ../qobject/json-parser.c:580
    #6 0x55631310fe67 in json_message_process_token ../qobject/json-streamer.c:92
    #7 0x5563131210b7 in json_lexer_feed_char ../qobject/json-lexer.c:313
    #8 0x556313121662 in json_lexer_feed ../qobject/json-lexer.c:350
    #9 0x5563131101e9 in json_message_parser_feed ../qobject/json-streamer.c:121
    #10 0x5563130cb81e in qmp_fd_receive ../tests/qtest/libqtest.c:614
    #11 0x5563130cba2b in qtest_qmp_receive_dict ../tests/qtest/libqtest.c:636
    #12 0x5563130cb939 in qtest_qmp_receive ../tests/qtest/libqtest.c:624
    #13 0x5563130cbe0d in qtest_vqmp ../tests/qtest/libqtest.c:715
    #14 0x5563130cc40f in qtest_qmp ../tests/qtest/libqtest.c:756
    #15 0x5563130c5623 in pwm_qom_get ../tests/qtest/npcm7xx_pwm-test.c:180
    #16 0x5563130c595e in pwm_get_duty ../tests/qtest/npcm7xx_pwm-test.c:210
    #17 0x5563130c7529 in test_toggle ../tests/qtest/npcm7xx_pwm-test.c:447

Reported-by: Euler Robot <euler.robot@huawei.com>
Message-Id: <20210115075634.717909-1-ganqixin@huawei.com>
Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>