swaybar: ensure correct init order for status_line

`$WAYLAND_SOCKET` is unset by `wl_display_connect` after it has
successfully connected to the wayland socket.

However, subprocesses spawned by swaybar (status-command) don't have
access to waybar's fds as $WAYLAND_SOCKET is O_CLOEXEC. This means any
status command which itself tries to connect to wayland will fail if
this environment variable is set.

Reorder display and status-command initialization so that this variable
is not set and add an assert so we can enforce this invariant in future.

swaybar/bar.c

@@ -413,10 +413,6 @@ bool bar_setup(struct swaybar *bar, const char *socket_path) {
 	if (!ipc_initialize(bar)) {
 		return false;
 	}
-	if (bar->config->status_command) {
-		bar->status = status_line_init(bar->config->status_command);
-		bar->status->bar = bar;
-	}
 
 	bar->display = wl_display_connect(NULL);
 	if (!bar->display) {
@@ -445,6 +441,11 @@ bool bar_setup(struct swaybar *bar, const char *socket_path) {
 		assert(pointer->cursor_surface);
 	}
 
+	if (bar->config->status_command) {
+		bar->status = status_line_init(bar->config->status_command);
+		bar->status->bar = bar;
+	}
+
 #if HAVE_TRAY
 	if (!bar->config->tray_hidden) {
 		bar->tray = create_tray(bar);

swaybar/status_line.c

@@ -1,4 +1,5 @@
 #define _POSIX_C_SOURCE 200809L
+#include <assert.h>
 #include <fcntl.h>
 #include <sys/ioctl.h>
 #include <json.h>
@@ -153,6 +154,8 @@ struct status_line *status_line_init(char *cmd) {
 		exit(1);
 	}
 
+	assert(!getenv("WAYLAND_SOCKET") && "display must be initialized before "
+		" starting `status-command`; WAYLAND_SOCKET should not be set");
 	status->pid = fork();
 	if (status->pid == 0) {
 		dup2(pipe_read_fd[1], STDOUT_FILENO);