Ensure keyread_tmp variable is assigned.

In the case of calcuating cost for a ref access for which there exists
a usable range, the variable keyread_tmp would always be 0.
If there is also another index that could be used as a filter, the cost
of that filter would be wrong.
In many cases 'the worst_seeks optimzation' would disable the filter
from getting used, which is why this bug has not been noticed before.

The next commit, which allows one to disable worst_seeks, will have a
test case for this bug.

sql/sql_select.cc

@@ -8219,6 +8219,9 @@ best_access_path(JOIN      *join,
                 trace_access_idx.add("used_range_estimates", true);
                 tmp= adjust_quick_cost(table->opt_range[key].cost,
                                        table->opt_range[key].rows);
+                keyread_tmp= table->file->keyread_time(key, 1,
+                                                       table->opt_range[key].
+                                                       rows);
                 goto got_cost;
               }
               else