g_content_type_guess segfaults when passed an empty data buffer on Mac OS
See also: https://github.com/fontforge/fontforge/issues/3553 for more info.
This case in particular was from calling
g_content_type_guess with a non NULL
data buffer, but where the
data_size is 0.
The issue is here: https://gitlab.gnome.org/GNOME/glib/blob/master/gio/gosxcontenttype.m#L539
xdg_mime_get_mime_type_for_data is called with
NULL. However, if
len is 0, then
result_prio is unconditionally set to 100:
Leading to the segfault.