do not call signal-unsafe function inside sighanlder

die() calls vprintf, fputc and exit; none of these are
async-signal-safe, see `man 7 signal-safety`.

dwm.c

@@ -1541,6 +1541,8 @@ setup(void)
 	Atom utf8string;
 
 	/* clean up any zombies immediately */
+	if (signal(SIGCHLD, sigchld) == SIG_ERR)
+		die("can't install SIGCHLD handler:");
 	sigchld(0);
 
 	/* init screen */
@@ -1638,8 +1640,6 @@ showhide(Client *c)
 void
 sigchld(int unused)
 {
-	if (signal(SIGCHLD, sigchld) == SIG_ERR)
-		die("can't install SIGCHLD handler:");
 	while (0 < waitpid(-1, NULL, WNOHANG));
 }