Set the garbd_exe variable to empty string to avoid warning about an...

Set the garbd_exe variable to empty string to avoid warning about an uninitialized variable when wsrep_provider is not initialized correctly, set to 'none' or when wsrep is switched off

mysql-test/mysql-test-run.pl

@@ -438,6 +438,7 @@ sub check_garbd_support() {
 }
 
 sub check_wsrep_support() {
+  $garbd_exe= "";
   if (have_wsrep()) {
     mtr_report(" - binaries built with wsrep patch");