MFC r211155: sh: Fix heap-based buffer overflow in pathname generation.

MFC r211155: sh: Fix heap-based buffer overflow in pathname generation.

The buffer for generated pathnames could be too small in some cases. It
happened to be always at least PATH_MAX long, so there was never an overflow
if the resulting pathnames would be usable.

This bug may be abused if a script subjects input from an untrusted source
to pathname generation, which a bad idea anyhow. Most shell scripts do not
work on untrusted data. secteam@ says no advisory is necessary.

PR:		bin/148733
Reported by:	Changming Sun snnn119 at gmail com

git-svn-id: svn://svn.freebsd.org/base/stable/8@211592 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f