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

[FX Coudert]

> We can reserve a few numbers for internal use - say, 2**31-1024 to 2**31-1
> (the maximum positive GFC_INTEGER_4).  Attempts to open or otherwise use
> these should be an error, and inquire(exist) could return false, like
> negative numbers right now.

It might be a stupid suggestion (I don't have access to the code right 
now): why not use negative numbers?

FX