Negative chances in Item Drop Set Data window, especially "Nothing Chance:"

Steps to recreate:

1. Quest->Misc Data->Item Drop Sets
2. Select any of the 256 item drop sets
3. Click 'Edit'
4. Type a negative number in "Nothing Chance:" text box
5. Observe negative percentage appears in adjacent percentage label
6. Click 'OK'
7. Click 'Edit' again
8. Observe negative number has been replaced with 16-bit two's complement

The chance text boxes on the two tabs clamp the negative chances to zero after clicking OK, but still update the percentage label with negative percentages in the meantime.

The "Nothing Chance:" string comes from the edititemdropset_dlg array in zquest.cpp

Negative percentages no longer occlude the dropdown.