When performing an RW modification of an array offset, the undefined
offset warning may call an error handler / OB callback, which may
destroy the array we're supposed to change. Detect this by temporarily
incrementing the reference count. If we find that the array has been
modified/destroyed in the meantime, we do nothing -- the execution
model here would be that the modification has happened on the destroyed
version of the array.
This replaces add_new with update for the RW case. This should not
be problematic for performance, as this branch throws a notice.
Alternatively add_new could also be replaced with add. I went with
update, because it makes $a[0] += 1 behavior the same as
$a[0] = $a[0] + 1.
