MDEV-8339 - Server crash during table cache eviction

TDC_element::free_tables_back() had pre-lfhash leftover code, which referenced
TDC_element::free_tables via TDC_element::share. This is not correct as share
may be NULL (newly inserted or to be removed), instead access free_tables
directly.

sql/table_cache.h

@@ -109,7 +109,7 @@ class TDC_element
 
   TABLE *free_tables_back()
   {
-    TABLE_list::Iterator it(share->tdc->free_tables);
+    TABLE_list::Iterator it(free_tables);
     TABLE *entry, *last= 0;
      while ((entry= it++))
        last= entry;