Avoid division by zero.

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@1664 f882894a-f735-0410-b71e-b25c423dba1c

plugins/check_ntp.c

@@ -608,7 +608,7 @@ double jitter_request(const char *host, int *status){
 		}
 	}
 
-	rval /= num_valid;
+	rval = num_valid ? rval / num_valid : -1.0;
 
 	close(conn);
 	free(peers);