Fix compiler warnings on wsrep_mysqld.cc

sql/wsrep_mysqld.cc:1897:9: warning: variable 'ret' is used

      uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]

    if (thd->is_error() && !wsrep_must_ignore_error(thd))

        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

sql/wsrep_mysqld.cc:1908:9: note: uninitialized use occurs here

    if (ret == 0)

        ^~~

sql/wsrep_mysqld.cc:1897:5: note: remove the 'if' if its

      condition is always false

    if (thd->is_error() && !wsrep_must_ignore_error(thd))

    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~