When using LEFT() function with a string that is without a charset,
the function crashes. This is because the function assumes that
the string has a charset, and tries to use it to calculate the
length of the string.

Two functions, UNHEX and WEIGHT_STRING, returned a string without
the charset being set to a not null value.

The fix is to set charset when calling val_str on these two functions.
Reviewed-by: Alexander Barkov <bar@mariadb.com>
Reviewed-by: Daniel Black <daniel@mariadb.org>