gnome-shell crashes when a cursor is not found

Hi,

As reported here, when a cursor theme lacks some icons, gnome-shell crashes.

Steps to reproduce:

• use gruppled_white_lite
• drag an icon from panel

Logs:

• Could not find cursor. Perhaps set XCURSOR_PATH?
• Unrecoverable failure in required component org.gnome.Shell.desktop
• Process 20825 (gnome-shell) of user 1000 dumped core.

Stack trace of thread 20825:
#0  0x00007f2e32c2325f raise (libpthread.so.0)
#1  0x00005560cd5b0912 n/a (gnome-shell)
#2  0x00007f2e32c233c0 __restore_rt (libpthread.so.0)
#3  0x00007f2e325469b6 n/a (libglib-2.0.so.0)
#4  0x00007f2e32546b9e g_log_default_handler (libglib-2.0.so.0)
#5  0x00005560cd5b0648 n/a (gnome-shell)
#6  0x00007f2e32547341 g_logv (libglib-2.0.so.0)
#7  0x00007f2e32547560 g_log (libglib-2.0.so.0)
#8  0x00007f2e3198d069 n/a (libmutter-3.so.0)
#9  0x00007f2e3198bf38 meta_cursor_tracker_get_sprite (libmutter-3.so.0)
#10 0x00007f2e3282475f shell_util_cursor_tracker_to_clutter (libgnome-shell.so)
#11 0x00007f2e2ff541c8 ffi_call_unix64 (libffi.so.6)
#12 0x00007f2e2ff53c2a ffi_call (libffi.so.6)
#13 0x00007f2e31cae854 n/a (libgjs.so.0)
#14 0x00007f2e31cb0059 n/a (libgjs.so.0)
#15 0x00007f2e2db4c34d n/a (libmozjs-60.so.0)
#16 0x00007ffe00000002 n/a (n/a)