cut: fix appending to unused cut buffer and support for wide characters

cut: fix appending to unused cut buffer and support for wide characters

Fix an if clause whose test assumes that a successful isupper test
returns 1 (grepping through the nvi sources, I haven't found other cases
where a character classification function is used with this assumption).

This fixes the following problem (on my i686 glibc based Gnu/Linux
system): Take an unused cut buffer, say a, and append something to
it, like "Ayy.  Now, :di b shows a cut buffer named (uppercase!) A.
Repeating "Ayy shows this bogus buffer twice instead of appending to it.
This affects both the 8-bit and wchar version.