Make the warning message during mariadb-backup SST more useful

When the SST log archive directory cannot be created or is not writable, the path is set to an empty string and a warning is logged. Unfortunately, the warning is logged only after the path has been changed, so, the following message is displayed:

WSREP_SST: [WARNING] The '' directory is not writtable
This PR swaps the order of logging and clearing the path so that the failing path is logged. The new output is:

WSREP_SST: [WARNING] The '/tmp/something_invalid' directory is not writable