With the fixes from !1638, it shouldn't be possible for this to happen
any more. However, non-positive sizes make no sense regardless, so if
this does somehow happen, let's make sure *something* reasonable happens.
The practical result of this assertion being hit is that we emit a
critical warning and then behave the same as if !1634 had been merged,
which is known to solve the issue for the submitter.
Signed-off-by: Simon McVittie <smcv@debian.org>
46165b52e5