fix crash in string_input_win

The bug appears only if one enters some text in the box before pressing KEY_UP (no crash if the text field is empty).

Problem was, that the code for calculating the menu height did not take into account the additional entry of the current text, only those from the actual history were counted for.

That triggered a popup in ui.cpp, which referenced items in an empty vector. This potential invalid access has been taken care of, too.

Reported here: http://smf.cataclysmdda.com/index.php?topic=5361

This might be the same as #5438, but my backtrace of the crash is different from the one listed there.

I simplified the code a bit, replaced access to `hmenu.w_y` with a constant 0, as that's the value it was  assigned in the previous line.