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

From: Paul Eggert <[email-masked]>
Date: Sat, 25 Jun 2016 01:08:58 +0200

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.