An VALUE_TYPE_EVAL score cannot overwrite an entry

If we want to store a value of type VALUE_TYPE_EVAL for
a given position and we found an already exsisting entry
for the same position then we skip.

We don't want to overwrite a more valuable score with a
lesser one. Note that also in case the exsisting entry is
of VALUE_TYPE_EVAL type the overwrite is unuseful because
we would store the same score again.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>