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.