This is a followup to the fix for Bug#12340997
get_interval_value() was trying to parse the input string,
looking for leading '-' while skipping whitespace.
The macro my_isspace() does not work for utf32 character set,
since my_charset_utf32_general_ci.ctype == NULL.

Solution: convert input to ASCII before parsing,
and use the character set of the returned ASCII string.