bug#23825: maint: avoid md5sum.c warning from bleeding-edge gcc's -Wstrict-overflow

[Pádraig Brady]

On 25/06/16 00:08, Paul Eggert wrote:
> I used full_write to simplify that, and also changed it to use just 
> full_write and not a mixture of full_write and stdio.

and clever reuse of argv[] for large operands.

thanks!
Pádraig