Null pointer dereference in handle_pointer_constraint_set_region

[emersion]

Ref 34e0cad

cc @Xyene

AddressSanitizer:DEADLYSIGNAL
=================================================================
==11662==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000018 (pc 0x5571603bf34a bp 0x7fff0d8ba750 sp 0x7fff0d8ba710 T0)
==11662==The signal is caused by a READ memory access.
==11662==Hint: address points to the zero page.
    #0 0x5571603bf34a in handle_pointer_constraint_set_region ../sway/input/cursor.c:837
    #1 0x7f8f4f4f3770 in wlr_signal_emit_safe ../subprojects/wlroots/util/signal.c:29
    #2 0x7f8f4f4b64b6 in pointer_constraint_commit ../subprojects/wlroots/types/wlr_pointer_constraints_v1.c:132
    #3 0x7f8f4f4b6510 in handle_surface_commit ../subprojects/wlroots/types/wlr_pointer_constraints_v1.c:140
    #4 0x7f8f4f4f3770 in wlr_signal_emit_safe ../subprojects/wlroots/util/signal.c:29
    #5 0x7f8f4f4d0c7e in surface_commit_pending ../subprojects/wlroots/types/wlr_surface.c:379
    #6 0x7f8f4f4d188d in surface_commit ../subprojects/wlroots/types/wlr_surface.c:448
    #7 0x7f8f4d645a8c  (/usr/lib/libffi.so.7+0x6a8c)
    #8 0x7f8f4d64501a  (/usr/lib/libffi.so.7+0x601a)
    #9 0x7f8f4e9d7f61  (/usr/lib/libwayland-server.so.0+0xcf61)
    #10 0x7f8f4e9d42db  (/usr/lib/libwayland-server.so.0+0x92db)
    #11 0x7f8f4e9d5fa9 in wl_event_loop_dispatch (/usr/lib/libwayland-server.so.0+0xafa9)
    #12 0x7f8f4e9d44e6 in wl_display_run (/usr/lib/libwayland-server.so.0+0x94e6)
    #13 0x557160360f7d in server_run ../sway/server.c:225
    #14 0x55716035d24c in main ../sway/main.c:409
    #15 0x7f8f4de40001 in __libc_start_main (/usr/lib/libc.so.6+0x27001)
    #16 0x55716032261d in _start (/home/simon/src/sway/build/sway/sway+0x31961d)