The tempfile functions all take pointers to tempfile
objects, but do not check whether the argument is NULL.
This isn't a big deal in practice, since the lifetime of any
tempfile object is defined to last for the whole program. So
even if we try to call delete_tempfile() on an
already-deleted tempfile, our "active" check will tell us
that it's a noop.
In preparation for transitioning to a new system that
loosens the "tempfile objects can never be freed" rule,
let's tighten up our active checks:
1. A NULL pointer is now defined as "inactive" (so it will
BUG for most functions, but works as a silent noop for
things like delete_tempfile).
2. Functions should always do the "active" check before
looking at any of the struct fields.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>