See the jifmux unit test from gst-plugins-bad. It's the exif buffer for the taglist generated in the test which causes this warning

Running suite(s): jifmux
==28035== Thread 2:
==28035== Syscall param write(buf) points to uninitialised byte(s)
==28035==    at 0x385FEC2F1D: ??? (syscall-template.S:82)
==28035==    by 0x385FE6DBA2: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1276)
==28035==    by 0x385FE6D819: new_do_write (fileops.c:530)
==28035==    by 0x385FE6DB0D: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1370)
==28035==    by 0x385FE63E5C: fwrite (iofwrite.c:45)
==28035==    by 0x67E423E: gst_file_sink_render (gstfilesink.c:653)
==28035==    by 0x4D71C74: gst_base_sink_render_object.clone.4 (gstbasesink.c:2872)
==28035==    by 0x4D740FE: gst_base_sink_queue_object_unlocked.clone.5 (gstbasesink.c:3153)
==28035==    by 0x4D74455: gst_base_sink_chain_unlocked.clone.6 (gstbasesink.c:3528)
==28035==    by 0x4D748AE: gst_base_sink_chain_main (gstbasesink.c:3566)
==28035==    by 0x4CA102B: gst_pad_chain_data_unchecked (gstpad.c:4231)
==28035==    by 0x4CA1872: gst_pad_push_data (gstpad.c:4463)
==28035==    by 0x4CA4718: gst_pad_push (gstpad.c:4685)
==28035==    by 0x4C2FBA9: gst_jif_mux_chain (gstjifmux.c:739)
==28035==    by 0x4CA102B: gst_pad_chain_data_unchecked (gstpad.c:4231)
==28035==    by 0x4CA1872: gst_pad_push_data (gstpad.c:4463)
==28035==    by 0x4CA4718: gst_pad_push (gstpad.c:4685)
==28035==    by 0x67B88B7: gst_jpegenc_term_destination (gstjpegenc.c:254)
==28035==    by 0x386F603C6E: jpeg_finish_compress (in /usr/lib/libjpeg.so.62.0.0)
==28035==    by 0x67B99FA: gst_jpegenc_chain (gstjpegenc.c:635)
==28035==  Address 0x5e379a3 is 483 bytes inside a block of size 15,833 alloc'd
==28035==    at 0x4A076B6: memalign (vg_replace_malloc.c:581)
==28035==    by 0x4A0770F: posix_memalign (vg_replace_malloc.c:709)
==28035==    by 0x4C77279: gst_buffer_try_new_and_alloc (gstbuffer.c:155)
==28035==    by 0x4CA08D3: gst_pad_alloc_buffer_full (gstpad.c:3020)
==28035==    by 0x4C305E2: gst_jif_mux_chain (gstjifmux.c:657)
==28035==    by 0x4CA102B: gst_pad_chain_data_unchecked (gstpad.c:4231)
==28035==    by 0x4CA1872: gst_pad_push_data (gstpad.c:4463)
==28035==    by 0x4CA4718: gst_pad_push (gstpad.c:4685)
==28035==    by 0x67B88B7: gst_jpegenc_term_destination (gstjpegenc.c:254)
==28035==    by 0x386F603C6E: jpeg_finish_compress (in /usr/lib/libjpeg.so.62.0.0)
==28035==    by 0x67B99FA: gst_jpegenc_chain (gstjpegenc.c:635)
==28035==    by 0x4CA102B: gst_pad_chain_data_unchecked (gstpad.c:4231)
==28035==    by 0x4CA1872: gst_pad_push_data (gstpad.c:4463)
==28035==    by 0x4CA4718: gst_pad_push (gstpad.c:4685)
==28035==    by 0x4D7A109: gst_base_src_loop (gstbasesrc.c:2503)
==28035==    by 0x4CCB5DF: gst_task_func (gsttask.c:318)
==28035==    by 0x3236A6EC0E: g_thread_pool_thread_proxy (gthreadpool.c:319)
==28035==    by 0x3236A6CD43: g_thread_create_proxy (gthread.c:1897)
==28035==    by 0x3860A068B9: start_thread (pthread_create.c:300)