Raise buffer size, fix checks for format_exit_helper_status.

This is probably not an exploitable bug, since you would need to have
errno be a large negative value in the unix pluggable-transport launcher
case.  Still, best avoided.

Fixes bug 9928; bugfix on 0.2.3.18-rc.
Give credit to bug reporter for 9928
Merge remote-tracking branch 'public/bug9928' into maint-0.2.3