buffer: fix copy meta reference debug log formatting

buffer: fix copy meta reference debug log formatting

Fixes the following valgrind error:
==616== Conditional jump or move depends on uninitialised value(s)
==616==    at 0x4900E34: gst_debug_print_object (gstinfo.c:1143)
==616==    by 0x49010B6: gst_info_printf_pointer_extension_func (gstinfo.c:1215)
==616==    by 0x4959FDB: __gst_printf_pointer_extension_serialize (printf-extension.c:47)
==616==    by 0x495A487: printf_postprocess_args (vasnprintf.c:258)
==616==    by 0x495A52C: __gst_vasnprintf (vasnprintf.c:290)
==616==    by 0x4959F8F: __gst_vasprintf (printf.c:154)
==616==    by 0x4901C1F: gst_debug_message_get (gstinfo.c:791)
==616==    by 0x4901C75: _gst_debug_log_preamble (gstinfo.c:1431)
==616==    by 0x4903208: gst_debug_log_default (gstinfo.c:1575)
==616==    by 0x49020BA: gst_debug_log_full_valist (gstinfo.c:624)
==616==    by 0x490211D: gst_debug_log_valist (gstinfo.c:656)
==616==    by 0x49021AD: gst_debug_log (gstinfo.c:533)
==616==    by 0x48DDC11: gst_buffer_copy_into (gstbuffer.c:693)
==616==    by 0x48DF5F1: gst_buffer_copy_with_flags (gstbuffer.c:727)
==616==    by 0x48DF640: gst_buffer_copy_deep (gstbuffer.c:756)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4038>