segfault with xapian 1.3.1

[Olly Betts]

Olly wrote on  IRC:
> bremner: ok, 1.2 explicitly no-oped skip_to() on an iterator at_end on
> trunk that code has been rewritten without that explicit check, and
> the iterator internals are NULL then i think restoring the check is
> reasonable, though I'm not sure if we actually promise that's defined
> behaviour if you want to work with 1.3.1, then checking against the
> end iterator before calling skip_to() will work for any version

I've put a NULL check in there, and also for the other iterator classes
which use NULL internals to signify being at the end - this only worked
for TermIterator in 1.2.x, but it seems reasonable to make it work in
general and a NULL pointer check isn't a big overhead.

On Thu, Sep 05, 2013 at 10:22:42PM -0300, David Bremner wrote:
> So, now we know what to fix.

This should work again in 1.3.2 (once it is out), but if you want to
support 1.3.1 then you need the check.

Cheers,
    Olly