Fix compiler warnings on gcc 6.x.

sql/sql_class.cc

@@ -2177,7 +2177,7 @@ bool THD::notify_shared_lock(MDL_context_owner *ctx_in_use,
 #ifdef WITH_WSREP
         {
           signalled|= mysql_lock_abort_for_thread(this, thd_table);
-          if (this && WSREP(this) && wsrep_thd_is_BF((void *)this, FALSE)) 
+          if (WSREP(this) && wsrep_thd_is_BF((void *)this, FALSE))
           {
             WSREP_DEBUG("remove_table_from_cache: %llu",
                         (unsigned long long) this->real_id);

sql/wsrep_mysqld.h

@@ -207,7 +207,7 @@ extern wsrep_seqno_t wsrep_locked_seqno;
    strcmp(wsrep_provider, WSREP_NONE))
 
 #define WSREP(thd) \
-  (WSREP_ON && wsrep && (thd && thd->variables.wsrep_on))
+  (WSREP_ON && wsrep && (thd->variables.wsrep_on))
 
 #define WSREP_CLIENT(thd) \
     (WSREP(thd) && thd->wsrep_client_thread)

sql/wsrep_sst.cc

@@ -814,7 +814,7 @@ static int sst_donate_mysqldump (const char*         addr,
                      "%s",
                      host, port, mysqld_port, mysqld_unix_port,
                      wsrep_defaults_file, uuid_str,
-                     (long long)seqno, bypass ? " "WSREP_SST_OPT_BYPASS : "");
+                     (long long)seqno, bypass ? " " WSREP_SST_OPT_BYPASS : "");
 
   if (ret < 0 || ret >= cmd_len)
   {
@@ -1125,7 +1125,7 @@ static int sst_donate_other (const char*   method,
                  wsrep_defaults_file, wsrep_defaults_group_suffix,
                  binlog_opt, binlog_opt_val,
                  uuid, (long long) seqno,
-                 bypass ? " "WSREP_SST_OPT_BYPASS : "");
+                 bypass ? " " WSREP_SST_OPT_BYPASS : "");
   my_free(binlog_opt_val);
 
   if (ret < 0 || ret >= cmd_len)