[patch, libgfortran] Fix PR20257 End of record occurs when writing large arrays

[Jerry DeLisle]

Jerry DeLisle wrote:
>>>
>>>
>> My first attempt at this was to use a negative number for unit.  IIRC 
>> there was specific existing code that generates an error for negative 
>> unit numbers.  I will explore this a bit and come back with something.
>>
> I must have been thinking of something else.  I just tried -1 for the 
> internal unit number and it regression tested fine.  So I think this 
> will do it.
> 
> For consistency, I think I will revise is_internal_unit to use the unit 
> number instead of a special bit.  If this all tests OK I will commit to 
> trunk and wait a while for 4.1 branch.

Disregard this last thought on changing is_internal_unit to use the unit number. 
  The reason it is using a special bit now is that when I tried this before, the 
test case negative_unit.f failed.  That test attempts to open a negative unit 
which then is mistaken for an internal unit and it all fails.

So the only change from the original patch is to set the default unit number for 
internal units to -1.  I will proceed with this commit later this evening.

> 
> Let me know if anyone has any other issues.
> 
> Regards,
> 
> Jerry
>