Clang warns on this code because it is memsetting over a vtable contained in a
struct in the best_positions array. The diagnostic text is:

mariadb/sql/sql_select.cc:24462:10: error: destination for this 'memset' call is
a pointer to class containing a dynamic class 'Duplicate_weedout_picker'; vtable
pointer will be overwritten [-Werror,-Wdynamic-class-memaccess]
  memset(best_positions, 0, sizeof(POSITION) * (table_count + 1));
  ~~~~~~ ^

Patch contributed by David Gow.