npcm7xx_adc-test: Fix memleak in adc_qom_set

The adc_qom_set 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 593280 byte(s) in 144 object(s) allocated from:
    #0 0x7f9a5e7e8d4e in __interceptor_calloc (/lib64/libasan.so.5+0x112d4e)
    #1 0x7f9a5e607a50 in g_malloc0 (/lib64/libglib-2.0.so.0+0x55a50)
    qemu#2 0x55b1bebf636b in qdict_new ../qobject/qdict.c:30
    qemu#3 0x55b1bec09699 in parse_object ../qobject/json-parser.c:318
    qemu#4 0x55b1bec0b2df in parse_value ../qobject/json-parser.c:546
    qemu#5 0x55b1bec0b6a9 in json_parser_parse ../qobject/json-parser.c:580
    qemu#6 0x55b1bec060d1 in json_message_process_token ../qobject/json-streamer.c:92
    qemu#7 0x55b1bec16a12 in json_lexer_feed_char ../qobject/json-lexer.c:313
    qemu#8 0x55b1bec16fbd in json_lexer_feed ../qobject/json-lexer.c:350
    qemu#9 0x55b1bec06453 in json_message_parser_feed ../qobject/json-streamer.c:121
    qemu#10 0x55b1bebc2d51 in qmp_fd_receive ../tests/qtest/libqtest.c:614
    qemu#11 0x55b1bebc2f5e in qtest_qmp_receive_dict ../tests/qtest/libqtest.c:636
    qemu#12 0x55b1bebc2e6c in qtest_qmp_receive ../tests/qtest/libqtest.c:624
    qemu#13 0x55b1bebc3340 in qtest_vqmp ../tests/qtest/libqtest.c:715
    qemu#14 0x55b1bebc3942 in qtest_qmp ../tests/qtest/libqtest.c:756
    qemu#15 0x55b1bebbd64a in adc_qom_set ../tests/qtest/npcm7xx_adc-test.c:127
    qemu#16 0x55b1bebbd793 in adc_write_input ../tests/qtest/npcm7xx_adc-test.c:140
    qemu#17 0x55b1bebbdf92 in test_convert_external ../tests/qtest/npcm7xx_adc-test.c:246

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Message-id: 20210118065627.79903-1-ganqixin@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

tests/qtest/npcm7xx_adc-test.c

@@ -129,6 +129,7 @@ static void adc_qom_set(QTestState *qts, const ADC *adc,
             path, name, value);
     /* The qom set message returns successfully. */
     g_assert_true(qdict_haskey(response, "return"));
+    qobject_unref(response);
 }
 
 static void adc_write_input(QTestState *qts, const ADC *adc,