valgrind test: test_appsrc_period_with_custom_segment is flaky

Introduced in !663 (merged)

==16594== Thread 2 appsrc0:src:
==16594== Invalid read of size 8
==16594==    at 0x404745: appsrc_pad_probe (appsrc.c:697)
==16594==    by 0x48CD96D: probe_hook_marshal (gstpad.c:3637)
==16594==    by 0x4A2A7A5: g_hook_list_marshal (ghook.c:672)
==16594==    by 0x48CD01D: do_probe_callbacks (gstpad.c:3800)
==16594==    by 0x48CF55A: gst_pad_chain_data_unchecked (gstpad.c:4379)
==16594==    by 0x48D1520: gst_pad_push_data (gstpad.c:4655)
==16594==    by 0x48D8412: gst_pad_push (gstpad.c:4774)
==16594==    by 0x4FB726C: gst_base_src_loop (gstbasesrc.c:3030)
==16594==    by 0x490644E: gst_task_func (gsttask.c:328)
==16594==    by 0x4A64753: g_thread_pool_thread_proxy (gthreadpool.c:308)
==16594==    by 0x4A64011: g_thread_proxy (gthread.c:805)
==16594==    by 0x4F4D4E1: start_thread (pthread_create.c:479)
==16594==    by 0x4C6E6A2: clone (clone.S:95)
==16594==  Address 0x6e85c60 is 0 bytes inside a block of size 280 free'd
==16594==    at 0x483EA0C: free (vg_replace_malloc.c:538)
==16594==    by 0x4A4049C: g_free (gmem.c:192)
==16594==    by 0x4A590E3: g_slice_free1 (gslice.c:1135)
==16594==    by 0x48F573E: gst_buffer_unref (gstbuffer.h:446)
==16594==    by 0x48F573E: _gst_sample_free (gstsample.c:86)
==16594==    by 0x404F50: gst_sample_unref (gstsample.h:118)
==16594==    by 0x404F50: test_appsrc_period_with_custom_segment (appsrc.c:793)
==16594==    by 0x49A9984: tcase_run_tfun_fork (check_run.c:465)
==16594==    by 0x49A9984: srunner_iterate_tcase_tfuns (check_run.c:237)
==16594==    by 0x49A9984: srunner_run_tcase (check_run.c:377)
==16594==    by 0x49A9984: srunner_iterate_suites (check_run.c:205)
==16594==    by 0x49A9984: srunner_run_tagged (check_run.c:740)
==16594==    by 0x499DEEE: gst_check_run_suite (gstcheck.c:1086)
==16594==    by 0x40371A: main (appsrc.c:1048)
==16594==  Block was alloc'd at
==16594==    at 0x483D80B: malloc (vg_replace_malloc.c:307)
==16594==    by 0x4A403A8: g_malloc (gmem.c:99)
==16594==    by 0x4A589D5: g_slice_alloc (gslice.c:1024)
==16594==    by 0x4894A4B: gst_buffer_new (gstbuffer.c:850)
==16594==    by 0x4895CE2: gst_buffer_new_allocate (gstbuffer.c:898)
==16594==    by 0x404EAE: test_appsrc_period_with_custom_segment (appsrc.c:777)
==16594==    by 0x49A9984: tcase_run_tfun_fork (check_run.c:465)
==16594==    by 0x49A9984: srunner_iterate_tcase_tfuns (check_run.c:237)
==16594==    by 0x49A9984: srunner_run_tcase (check_run.c:377)
==16594==    by 0x49A9984: srunner_iterate_suites (check_run.c:205)
==16594==    by 0x49A9984: srunner_run_tagged (check_run.c:740)
==16594==    by 0x499DEEE: gst_check_run_suite (gstcheck.c:1086)
==16594==    by 0x40371A: main (appsrc.c:1048)
==16594== 

Full log: https://gitlab.freedesktop.org/ystreet/gst-plugins-base/-/jobs/4521631

cc @seungha.yang