Fix GCC 9 -Wmaybe-uninitialized

Always initialize ScopedStatementReplication::saved_binlog_format,
so that GCC cannot emit a bogus warning about
ScopedStatementReplication::~ScopedStatementReplication() using the
variable.

The code was originally introduced in
commit d998da0.

sql/sql_class.h

@@ -1,6 +1,6 @@
 /*
    Copyright (c) 2000, 2016, Oracle and/or its affiliates.
-   Copyright (c) 2009, 2017, MariaDB Corporation.
+   Copyright (c) 2009, 2019, MariaDB Corporation.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -6761,20 +6761,21 @@ void dbug_serve_apcs(THD *thd, int n_calls);
 class ScopedStatementReplication
 {
 public:
-  ScopedStatementReplication(THD *thd) : thd(thd)
-  {
-    if (thd)
-      saved_binlog_format= thd->set_current_stmt_binlog_format_stmt();
-  }
+  ScopedStatementReplication(THD *thd) :
+    saved_binlog_format(thd
+                        ? thd->set_current_stmt_binlog_format_stmt()
+                        : BINLOG_FORMAT_MIXED),
+    thd(thd)
+  {}
   ~ScopedStatementReplication()
   {
     if (thd)
       thd->restore_stmt_binlog_format(saved_binlog_format);
   }
 
 private:
-  enum_binlog_format saved_binlog_format;
-  THD *thd;
+  const enum_binlog_format saved_binlog_format;
+  THD *const thd;
 };
 
 #endif /* MYSQL_SERVER */