Make sure that argument passed to strncmp is non-NULL

Hello!

g_file_info_get_content_type() may return NULL which unfortunately isn't documented. I've filed the issue for the same on GNOME gitlab https://gitlab.gnome.org/GNOME/glib/-/issues/2124

So such returned pointer should not be passed to strncmp() without making sure that it's non-NULL.

But in src/image_list.c Line 766 we pass content_type returned from g_file_info_get_content_type directly to strncmp(). I think we can fix this as:

-        if (strncmp (content_type, "image/", 6) == 0)
+        if (content_type && (strncmp (content_type, "image/", 6) == 0))

I'm a long time XFCE user (4+ years) and always wanted to contribute. May I submit the MR for this issue?

Thanks!