Fix crash from ipc_json_describe_workspace

destroying workspace '10'
Error: Signal 11. Printing backtrace
Backtrace: sway(error_handler+0x2f) [0x41478f]
Backtrace: /usr/lib/libc.so.6(+0x33680) [0x7f06d9a99680]
Backtrace: /usr/lib/libc.so.6(strlen+0x2a) [0x7f06d9ae70ca]
Backtrace: /usr/lib/libc.so.6(__strdup+0xe) [0x7f06d9ae6dfe]
Backtrace: /usr/lib/libjson-c.so.2(json_object_new_string+0x39) [0x7f06da01ffe9]
Backtrace: sway(ipc_json_describe_workspace+0xd8) [0x410408]
Backtrace: sway(ipc_event_workspace+0x38) [0x410618]
Backtrace: sway(set_focused_container+0x190) [0x40daf0]
Backtrace: sway(workspace_switch+0x1a1) [0x4142b1]
Backtrace: sway() [0x4096c6]
Backtrace: sway(handle_command+0x178) [0x405eb8]
Backtrace: sway() [0x40e6ee]
Backtrace: /usr/lib/libwlc.so.0(+0x19a1b) [0x7f06da6f0a1b]
Backtrace: /usr/lib/libwlc.so.0(+0x11b0c) [0x7f06da6e8b0c]
Backtrace: /usr/lib/libwayland-server.so.0(wl_event_loop_dispatch+0x72) [0x7f06d9e13242]
Backtrace: /usr/lib/libwayland-server.so.0(wl_display_run+0x25) [0x7f06d9e119a5]
Backtrace: /usr/lib/libwlc.so.0(wlc_run+0xfc) [0x7f06da6fda1c]
Backtrace: sway(main+0x25f) [0x4125cf]
Backtrace: /usr/lib/libc.so.6(__libc_start_main+0xf0) [0x7f06d9a86610]
Backtrace: sway(_start+0x29) [0x405bf9]

sway/ipc-server.c

@@ -493,6 +493,10 @@ bool ipc_send_reply(struct ipc_client *client, const char *payload, uint32_t pay
 }
 
 json_object *ipc_json_describe_workspace(swayc_t *workspace) {
+	if (!sway_assert(workspace, "Workspace must not be NULL")) {
+		return NULL;
+	}
+
 	int num = isdigit(workspace->name[0]) ? atoi(workspace->name) : -1;
 	json_object *object = json_object_new_object();
 	json_object *rect = json_object_new_object();