MDEV-29706 : SIGSEGV in wsrep_TOI_begin on non-Galera builds

Do not allow setting wsrep_on=ON if no provider is set.

mysql-test/suite/sys_vars/r/wsrep_on_without_provider.result

+SET GLOBAL wsrep_on=ON;
+ERROR HY000: WSREP (galera) can't be enabled if the wsrep_provider is unset or set to 'none'
+SELECT @@global.wsrep_on;
+@@global.wsrep_on
+0

mysql-test/suite/sys_vars/t/wsrep_on_basic.test

---source include/have_wsrep.inc
-
---echo #
---echo # wsrep_on
---echo #
-
---echo # save the initial values
-SET @wsrep_on_global_saved = @@global.wsrep_on;
-SET @wsrep_on_session_saved = @@session.wsrep_on;
-
---echo # default
-SELECT @@global.wsrep_on;
-SELECT @@session.wsrep_on;
-
---echo
---echo # scope and valid values
-SET @@global.wsrep_on=OFF;
-SELECT @@global.wsrep_on;
-SET @@global.wsrep_on=ON;
-SELECT @@global.wsrep_on;
-
-SET @@session.wsrep_on=OFF;
-SELECT @@session.wsrep_on;
-SET @@session.wsrep_on=ON;
-SELECT @@session.wsrep_on;
-SET @@session.wsrep_on=default;
-SELECT @@session.wsrep_on;
-
---echo
---echo # invalid values
---error ER_WRONG_VALUE_FOR_VAR
-SET @@global.wsrep_on=NULL;
---error ER_WRONG_VALUE_FOR_VAR
-SET @@global.wsrep_on='junk';
---error ER_WRONG_VALUE_FOR_VAR
-SET @@session.wsrep_on=NULL;
---error ER_WRONG_VALUE_FOR_VAR
-SET @@session.wsrep_on='junk';
-
---echo
---echo # restore the initial values
-SET @@global.wsrep_on = @wsrep_on_global_saved;
-SET @@session.wsrep_on = @wsrep_on_session_saved;
-
---echo # End of test

mysql-test/suite/sys_vars/t/wsrep_on_without_provider.test

+--source include/not_embedded.inc
+
+#
+# @@global.wsrep_on is not allowed if there
+# is no wsrep_provider
+#
+--error ER_WRONG_ARGUMENTS
+SET GLOBAL wsrep_on=ON;
+SELECT @@global.wsrep_on;
\ No newline at end of file

sql/wsrep_var.cc

@@ -70,6 +70,12 @@ bool wsrep_on_check(sys_var *self, THD* thd, set_var* var)
 	    " innodb_lock_schedule_algorithm=FCFS and restart.", MYF(0));
     return true;
   }
+  if (new_wsrep_on && !WSREP_PROVIDER_EXISTS)
+  {
+    my_message(ER_WRONG_ARGUMENTS, "WSREP (galera) can't be enabled "
+               "if the wsrep_provider is unset or set to 'none'", MYF(0));
+    return true;
+  }
   return false;
 }