Fix UBSAN failure: sql_select.h:982:7: load of value ... not valid for type bool

This is 11.0 part of the fix: in 11.0, get_costs_for_tables() calls
best_access_path() for all possible tables, for each call it saves
a POSITION object with the access method and "loose_scan_pos"
POSITION object.
The latter is saved even if there is no possible LooseScan plan. Saving
is done by copying POSITION objects which may generate a spurious
UBSan error.

sql/sql_select.cc

@@ -449,6 +449,7 @@ POSITION::POSITION()
   key= 0;
   forced_index= 0;
   use_join_buffer= 0;
+  firstmatch_with_join_buf= false;
   sj_strategy= SJ_OPT_NONE;
   n_sj_tables= 0;
   spl_plan= 0;