get_interval_value() was trying to parse the input string,
looking for leading '-' while skipping whitespace.
The macro my_isspace() does not work for utf16 character set,
since my_charset_utf16_general_ci.ctype == NULL.

Solution: convert input to ASCII before parsing.


mysql-test/r/ctype_utf16.result:
  New test case.
mysql-test/t/ctype_utf16.test:
  New test case.
sql/item_timefunc.cc:
  Use val_string_ascii() rather than val_string()
  so that we can safely use my_isspace() for skipping whitespace.