dialog: Fix crash in debugging function (debug_main_timer_list())

DBG:dialog:debug_main_timer_list: testing forward loop with visited = 1
DBG:dialog:debug_main_timer_list: testing backward loop with visited = 2
DBG:dialog:insert_dlg_timer_unsafe: inserting 0x7f303dba8d80 for 7205
DBG:dialog:debug_main_timer_list: testing forward loop with visited = 1
ERROR:dialog:debug_main_timer_list: Detected something wrong with main timer list on forward linking for entry 0x7f303dba8d80
DBG:core:handle_sigs: OpenSIPS exit status = 134
INFO:core:handle_sigs: child process 139408 exited by a signal 6
(cherry picked from commit 4c6a8ec)

modules/dialog/dlg_timer.c

@@ -103,9 +103,11 @@ void debug_detached_timer_list(struct dlg_tl *detached)
 /* assumed to be always called under timer lock */
 void debug_main_timer_list(void)
 {
+	static int visited;
+
 	struct dlg_tl *start,*finish;
-	int visited=1;
 
+	visited++;
 	start = finish = &(d_timer->first);
 	LM_DBG("testing forward loop with visited = %d\n",visited);