v1.6.1 - bugfix for nil values in processItemCost

This bugfix aims to fix the following script error from nil values being passed to processItemCost.
Script Error: [string "scripts/real_encumbrance.lua"]:94: attempt to perform arithmetic on global 'nTrimmedItemCost' (a nil value)

The reason for this error (in my testing) was because of items with costs entered in a format like '1gp-100gp'.
This sort of entry will now be ignored (along with any other item with "-" in its cost field).

For more information, see the previous release:
https://github.com/bmos/FG-PFRPG-TotalEncumbrance/releases/tag/v1.6.0