Fix Coverity warning for possible NULL ptr dereference

Add check for return value from strtok()

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/pidfile.c

@@ -36,6 +36,9 @@ static void pidfile_callback(void *UNUSED(arg), int fd, int UNUSED(events))
 			continue;
 
 		basename = strtok(ev->name, ".");
+		if (!basename)
+			continue;
+
 		svc = svc_find_by_nameid(basename, 1);
 		if (!svc)
 			continue;