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).
1. fix useless correction of dec_arg *after it was used*
2. use DECIMAL_NOT_SPECIFIED instead of NOT_FIXED_DEC, because the latter
is defined conditionally and has a wrong value in plugins
There where several reasons why the test failed:
- Constructors for Field_double and Field_float changed an argument
to the constructor instead of a the correct class variable.
- gcc 7.5.0 produced wrong code when inlining Field_double constructor
into Field_test_double constructor.
Fixed by changing the correct class variable and make the constructors
not inline to go around the gcc bug.
