rev: Check for wchar conversion errors

rev: Check for wchar conversion errors

Commit c9cc84621ca98ef85499e83ca56f05f12055f193 introduced a regression
where only the actual EOF is handled, not other error conditions
returning WEOF. This leads to an infinite loop upon encountering
conversion errors. For example (using LC_CTYPE="en_US.UTF-8"):

$ printf '\x80' | rev

Signed-off-by: Tim Hallmann <tim@t8w.de>