wsrep_sst_auth: fix a memory leak

wsrep_sst_auth_init() is always invoked with value==wsrep_sst_auth.
Old code was leaking value, because it was never freed.

sql/wsrep_sst.cc

@@ -182,8 +182,8 @@ bool wsrep_sst_auth_update (sys_var *self, THD* thd, enum_var_type type)
 
 void wsrep_sst_auth_init (const char* value)
 {
-    if (wsrep_sst_auth == value) wsrep_sst_auth = NULL;
-    if (value) sst_auth_real_set (value);
+    DBUG_ASSERT(wsrep_sst_auth == value);
+    sst_auth_real_set (wsrep_sst_auth);
 }
 
 bool  wsrep_sst_donor_check (sys_var *self, THD* thd, set_var* var)