cmd/6c, cmd/8c: fix stack allocated Biobuf leaking at exit

Fixes issue 5085.

{6,8}c/swt.c allocates a third Biobuf in automatic memory which is not terminated at the end of the function. This causes the buffer to be 'in use' when the batexit handler fires, confusing valgrind.

Huge thanks to DMorsing for the diagnosis.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://codereview.appspot.com/7844044