MDEV-21263: Fix -Wclass-memaccess

Since commit f52bf92014 the type
Sql_sort is non-trivial, because it includes a data member
Bounds_checked_array<SORT_FIELD> local_sortorder.
There still is no vtable, so memset() is safe to invoke, but
we must add a cast to silence a warning in GCC 8 or later.

sql/sql_sort.h

@@ -300,7 +300,7 @@ public:
 
   Sort_param()
   {
-    memset(this, 0, sizeof(*this));
+    memset(reinterpret_cast<void*>(this), 0, sizeof(*this));
   }
   void init_for_filesort(uint sortlen, TABLE *table,
                          ha_rows maxrows, bool sort_positions);