MDEV-10186: mysqld crash when runtime setting wsrep_cluster_address without wsrep_on=ON

On wsrep_cluster_address update, node restarts the replication
and attempts to connect to the new address. In this process it
makes a call to wsrep provider's connect API, which could lead
to segfault if wsrep provider is not loaded (wsrep_on=OFF).

Fixed by making sure that it proceeds only if a provider is
loaded.

mysql-test/suite/wsrep/r/mdev_10186.result

+#
+# MDEV-10186: mysqld crash when runtime setting
+# wsrep_cluster_address without wsrep_on=ON
+#
+SELECT @@wsrep_on;
+@@wsrep_on
+0
+SELECT @@GLOBAL.wsrep_provider;
+@@GLOBAL.wsrep_provider
+/usr/lib/galera/libgalera_smm.so
+SET @@GLOBAL.wsrep_cluster_address='gcomm://';

mysql-test/suite/wsrep/t/mdev_10186.opt

+--wsrep-provider=$WSREP_PROVIDER --wsrep-cluster-address=gcomm:// --wsrep-on=0

mysql-test/suite/wsrep/t/mdev_10186.test

+--source include/have_wsrep_provider.inc
+--source include/have_binlog_format_row.inc
+
+--echo #
+--echo # MDEV-10186: mysqld crash when runtime setting
+--echo # wsrep_cluster_address without wsrep_on=ON
+--echo #
+
+SELECT @@wsrep_on;
+SELECT @@GLOBAL.wsrep_provider;
+SET @@GLOBAL.wsrep_cluster_address='gcomm://';
+

sql/wsrep_mysqld.cc

@@ -881,6 +881,9 @@ bool wsrep_start_replication()
 {
   wsrep_status_t rcode;
 
+  /* wsrep provider must be loaded. */
+  DBUG_ASSERT(wsrep);
+
   /*
     if provider is trivial, don't even try to connect,
     but resume local node operation

sql/wsrep_var.cc

@@ -348,7 +348,16 @@ bool wsrep_cluster_address_check (sys_var *self, THD* thd, set_var* var)
 
 bool wsrep_cluster_address_update (sys_var *self, THD* thd, enum_var_type type)
 {
-  bool wsrep_on_saved= thd->variables.wsrep_on;
+  bool wsrep_on_saved;
+
+  /* Do not proceed if wsrep provider is not loaded. */
+  if (!wsrep)
+  {
+    WSREP_INFO("wsrep provider is not loaded, can't re(start) replication.");
+    return false;
+  }
+
+  wsrep_on_saved= thd->variables.wsrep_on;
   thd->variables.wsrep_on= false;
 
   /* stop replication is heavy operation, and includes closing all client