Redshift reads run every simple-query cell through decode_pg_text_wkb
with an unknown column type. The probe sliced hex[..2] before the
is_ascii() guard, so any cell starting with a 3-4 byte UTF-8 character
with an even byte length >= 10 panicked on a non-char boundary and
aborted the process under panic = abort (Windows BEX64 in dbx.exe).
Check is_ascii() first so ordinary multibyte text passes through.
Closes#7958