url: don't use bad offset in tld_check_name to show error

libidn's tld_check_lz returns an error offset of the first character
that it failed to process, however that offset is not a byte offset and
may not even be in the locale encoding therefore we can't use it to show
the user the character that failed to process.

Bug: https://github.com/curl/curl/issues/731
Reported-by: Karlson2k