Do not use stderr for log file and don't call log_close when not needed.

log.c

@@ -28,7 +28,7 @@
 #include "tmux.h"
 
 /* Log file, if needed. */
-FILE		*log_file = stderr;
+FILE		*log_file;
 
 /* Debug level. */
 int		 log_level = 0;
@@ -63,7 +63,7 @@ log_open(int level, const char *path)
 void
 log_close(void)
 {
-	if (log_file != stderr)
+	if (log_file != NULL)
 		fclose(log_file);
 
 	event_set_log_callback(NULL);
@@ -75,6 +75,9 @@ log_vwrite(const char *msg, va_list ap)
 {
 	char	*fmt;
 
+	if (log_file == NULL)
+		return;
+
 	if (asprintf(&fmt, "%s\n", msg) == -1)
 		exit(1);
 	if (vfprintf(log_file, fmt, ap) == -1)

tmux.c

@@ -70,7 +70,6 @@ logfile(const char *name)
 {
 	char	*path;
 
-	log_close();
 	if (debug_level > 0) {
 		xasprintf(&path, "tmux-%s-%ld.log", name, (long) getpid());
 		log_open(debug_level, path);