Buffer under-read in Text::CSV_XS trailing blank stripping

Text::CSV_XS versions before 1.65 for Perl read before the start of an all-blank field under allow_whitespace in strip_trail_whitespace. Under allow_whitespace, strip_trail_whitespace removes trailing spaces and tabs from each unquoted field without stopping at the start of the field. For a field of only blanks it reads the byte before the field's heap buffer. If that byte is also a space or tab, it may be zeroed and the walk continue backwards, depending on what the allocator placed there. Leading blanks are normally skipped before a field is stored, but an all-blank field still arrives after a leading escape_char that differs from quote_char, or as a quoted field in getline when its line ends in two CRs.