check_http: Fix -C/--certificate option handling

The support for specifying the desired SSL protocol version via an
optional -S/--ssl argument broke the -C/--certificate option.  This is
fixed now.

plugins/check_http.c

@@ -295,7 +295,7 @@ process_arguments (int argc, char **argv)
       usage4 (_("Invalid option - SSL is not available"));
 #endif
       use_ssl = TRUE;
-      if (optarg == NULL)
+      if (optarg == NULL || c != 'S')
         ssl_version = 0;
       else {
         ssl_version = atoi(optarg);