fix "slice bounds out of range" in cmd-delete-word-back

[dazzywi]

Hello, while trying to edit file names with Cyrillic characters in them, I encountered error when performing cmd-delete-word-back action to remove words:

runtime error: slice bounds out of range [:18] with capacity 16 

To reproduce you can create two folders, one named test test test and тест тест тест, position your cursor at the end of file name (like this тест тест тест| as denoted by |), and try to delete one word back.

You may also need to bind this action to some key beforehand in lfrc, like this cmap <c-w> cmd-delete-word-back.

Prior to this commit, after deleting one word back in file names:
test test test will become test test
тест тест тест will crash lf, when it is expected to become тест тест

[joelim-work]

Thanks for the bug fix!