gstvalue: fix crash when struct to string for allocation params

when gst_buffer_pool_config_set_allocator (config, alloc, NULL); gst_structure_to_string or GST_DEBUG (pool, "config %" GST_PTR_FORMAT, config) will crash, this patch fix that

i test with following cmd in latest version. issue not found in 1.22, it is strange. in latest version, i build gstreamer by static method.

gst-launch-1.0 videotestsrc ! video/x-raw,format=YUY2,width=1280,height=720 ! waylandsink

call flow
i found this line crash in waylandsink, that pass NULL for allocator params, when try print this structure
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gst-plugins-bad/ext/wayland/gstwaylandsink.c?ref_type=heads#L820
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gstreamer/gst/gstbufferpool.c?ref_type=heads#L608
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/blob/main/subprojects/gstreamer/gst/gstvalue.c?ref_type=heads#L8075