Crash when drag focus surface is destroyed

From https://bugzilla.redhat.com/show_bug.cgi?id=1625253

Steps to Reproduce:

1. Create few bookmarks in Nautilus
2. Drag one of them and hover with it over "Other Locations" section

Backtrace:

#0  0x00007fee47a8f374 in meta_wayland_surface_drag_dest_focus_out (surface=0x3eb6c60) at ../../src/wayland/meta-wayland-surface.c:1368
#1  0x00007fee47a82010 in meta_wayland_drag_grab_set_focus (drag_grab=0x1d86900, surface=0x0) at ../../src/wayland/meta-wayland-data-device.c:779
#2  0x00007fee47a82282 in drag_grab_focus (grab=<optimized out>, surface=<optimized out>) at ../../src/wayland/meta-wayland-data-device.c:833
#3  0x00007fee47a856a6 in sync_focus_surface (pointer=pointer@entry=0x1aed5a0) at ../../src/wayland/meta-wayland-pointer.c:243
#4  0x00007fee47a859fb in repick_for_event (pointer=0x1aed5a0, for_event=for_event@entry=0x0) at ../../src/wayland/meta-wayland-pointer.c:599
#5  0x00007fee47a85a70 in meta_wayland_pointer_repick (pointer=<optimized out>) at ../../src/wayland/meta-wayland-pointer.c:997
#6  0x00007fee47a88d63 in meta_wayland_seat_repick (seat=0x1dc9520) at ../../src/wayland/meta-wayland-seat.c:418
#7  0x00007fee47a7b367 in meta_wayland_compositor_repick (compositor=compositor@entry=0x7fee47d19ae0 <_meta_wayland_compositor>) at ../../src/wayland/meta-wayland.c:131
#8  0x00007fee47a8efdf in wl_surface_destructor (resource=<optimized out>) at ../../src/wayland/meta-wayland-surface.c:1216
#9  0x00007fee3cb8a17e in destroy_resource (element=element@entry=0x212a570, data=data@entry=0x0, flags=0) at src/wayland-server.c:688
#10 0x00007fee3cb8a1de in wl_resource_destroy (resource=0x212a570) at src/wayland-server.c:705
#11 0x00007fee47a8dd60 in wl_surface_destroy (client=<optimized out>, resource=<optimized out>) at ../../src/wayland/meta-wayland-surface.c:776
#12 0x00007fee437e103e in ffi_call_unix64 () from /lib64/libffi.so.6
#13 0x00007fee437e09ff in ffi_call () from /lib64/libffi.so.6
...