[Fontconfig] fontconfig crash for special bdf font

[Werner LEMBERG]

>> I see two possibilities on the FreeType side:
>>
>>   1. Document that NULL is a valid return value for `atom',
>>      corresponding to a property present in the font with a zero
>>      string as an argument – the return value
>>      `FT_Err_Invalid_Argument' of function `FT_Get_BDF_Property'
>>      indicates that the searched property isn't present in the font.
> 
> So FT_Get_BDF_Property will returns FT_Err_Invalid_Argument in this
> case? or returning NULL means it's there but it was just an empty
> string?

FT_Get_BDF_Property returns an error code, thus it never returns NULL.
For a valid property with an empty string, the field `prop.u.atom' is
set to NULL (and not to an empty C string).

> Anyway mentioning more details for the above thing in the doc sounds
> good to me.

Definitely :-)


    Werner