[poppler] [Patch] Fix crashes with the cairo backend

[Marco Pesenti Gritti]

This just copy a check from the splash backend. Given the render api,
we should really check to not be rendering outside the pixbuf, because
both scale and pixbuf are user provided and their size is not
guaranteed to match.
Now, the crashes looks like a consequence of the usual rounding issues
in evince, but as I said the check is really necessary.

Marco