libbio: fix Bprint bug
libbio: fix Bprint bug

Make Bprint work even when the amount of output
exceeds the available buffer space.

R=r
CC=golang-dev
http://codereview.appspot.com/1968041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go