window: Fix leak of event_creation_data struct

And also be sure to set its pointer to NULL by using g_clear_pointer()

8,830 (24 direct, 8,806 indirect) bytes in 1 blocks are
definitely lost in loss record 17,999 of 18,078
   at 0x4C2FA50: calloc (vg_replace_malloc.c:711)
   by 0x99CE981: g_malloc0 (gmem.c:124)
   by 0x99CEC64: g_malloc0_n (gmem.c:355)
   by 0x44CD43: show_new_event_widget (gcal-window.c:731)
   by 0x976FC57: ffi_call_unix64 (in /usr/lib64/libffi.so.6.0.2)
   by 0x976F6B9: ffi_call (in /usr/lib64/libffi.so.6.0.2)
   by 0x952703B: g_cclosure_marshal_generic (gclosure.c:1490)
   by 0x95258B9: g_closure_invoke (gclosure.c:804)
   by 0x9541D25: signal_emit_unlocked_R (gsignal.c:3635)
   by 0x954105C: g_signal_emit_valist (gsignal.c:3391)
   by 0x954170B: g_signal_emit_by_name (gsignal.c:3487)
   by 0x4324D8: show_popover_for_position (gcal-month-view.c:305)

https://bugzilla.gnome.org/show_bug.cgi?id=772379