swaynag: fix use-after-free in wl_display_dispatch

When destroying swaynag from within wl_display_dispatch, we cannot
disconnect the display as that will free the queue's event_list.
Free it after running the loop instead.

Fixes this use-after-free:
==7312==ERROR: AddressSanitizer: heap-use-after-free on address 0x612000000110 at pc 0x000000412a9f bp 0x7ffd4e811760 sp 0x7ffd4e811750
READ of size 8 at 0x612000000110 thread T0
    #0 0x412a9e in wl_list_empty ../common/list.c:206
    #1 0x7f5b58f0d42f in dispatch_queue src/wayland-client.c:1572
    #2 0x7f5b58f0d42f in wl_display_dispatch_queue_pending src/wayland-client.c:1815
    #3 0x40f465 in swaynag_run ../swaynag/swaynag.c:390
    #4 0x407576 in main ../swaynag/main.c:123
    #5 0x7f5b58bb9412 in __libc_start_main ../csu/libc-start.c:308
    #6 0x404a3d in _start (/opt/wayland/bin/swaynag+0x404a3d)

0x612000000110 is located 208 bytes inside of 320-byte region [0x612000000040,0x612000000180)
freed by thread T0 here:
    #0 0x7f5b594ab480 in free (/lib64/libasan.so.5+0xef480)
    #1 0x40faff in swaynag_destroy ../swaynag/swaynag.c:454
    #2 0x40cbb4 in layer_surface_closed ../swaynag/swaynag.c:82
    #3 0x7f5b583e1acd in ffi_call_unix64 (/lib64/libffi.so.6+0x6acd)

previously allocated by thread T0 here:
    #0 0x7f5b594aba50 in __interceptor_calloc (/lib64/libasan.so.5+0xefa50)
    #1 0x7f5b58f0c902 in wl_display_connect_to_fd src/wayland-private.h:236

(you need a wayland compiled with asan, my wl_list hack, or running
with valgrind to see this trace)

swaynag/swaynag.c

@@ -390,6 +390,10 @@ void swaynag_run(struct swaynag *swaynag) {
 			&& wl_display_dispatch(swaynag->display) != -1) {
 		// This is intentionally left blank
 	}
+
+	if (swaynag->display) {
+		wl_display_disconnect(swaynag->display);
+	}
 }
 
 void swaynag_destroy(struct swaynag *swaynag) {
@@ -449,8 +453,4 @@ void swaynag_destroy(struct swaynag *swaynag) {
 	if (swaynag->shm) {
 		wl_shm_destroy(swaynag->shm);
 	}
-
-	if (swaynag->display) {
-		wl_display_disconnect(swaynag->display);
-	}
 }