varnishevent bugfix: don't cancel the monitor thread if it's not running

bin/varnishevent/monitor.c

@@ -37,7 +37,7 @@
 #include "libvarnish.h"
 #include "vas.h"
 
-static int run;
+static int run = 0;
 
 static pthread_t monitor;
 static pthread_mutex_t stats_lock = PTHREAD_MUTEX_INITIALIZER;
@@ -94,9 +94,11 @@ MON_Output(void)
 void
 MON_Shutdown(void)
 {
-    run = 0;
-    AZ(pthread_cancel(monitor));
-    AZ(pthread_join(monitor, NULL));
+    if (run) {
+        run = 0;
+        AZ(pthread_cancel(monitor));
+        AZ(pthread_join(monitor, NULL));
+    }
     AZ(pthread_mutex_destroy(&stats_lock));
 }