Item_func_hex::fix_length_and_dec() evaluated a too short data type
for signed numeric arguments, which resulted in a 'Data too long for column'
error on CREATE..SELECT.

Fixing the code to take into account that a short negative
numer can produce a long HEX value: -1  -> 'FFFFFFFFFFFFFFFF'

Also fixing Item_func_hex::val_str_ascii_from_val_real().
Without this change, MTR test with HEX with negative float point arguments
failed on some platforms (aarch64, ppc64le, s390-x).