util.c: output function might override errno and thus affect perror()

Original patch by Raymond Cole with some modifications, thanks!

ref.
https://git.suckless.org/dwm/commit/fcb2476b693ca4c40ad32c7119e27bbeb856865c.html

util.c

@@ -1,4 +1,5 @@
 /* See LICENSE file for copyright and license details. */
+#include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -10,17 +11,16 @@ void
 die(const char *fmt, ...)
 {
 	va_list ap;
+	int saved_errno;
 
+	saved_errno = errno;
 	va_start(ap, fmt);
 	vfprintf(stderr, fmt, ap);
 	va_end(ap);
 
-	if (fmt[0] && fmt[strlen(fmt)-1] == ':') {
-		fputc(' ', stderr);
-		perror(NULL);
-	} else {
-		fputc('\n', stderr);
-	}
+	if (fmt[0] && fmt[strlen(fmt)-1] == ':')
+		fprintf(stderr, " %s", strerror(saved_errno));
+	fputc('\n', stderr);
 
 	exit(1);
 }