monitor-manager-xrandr: Be more robust when reading XRROutputInfos

We might get modes in XRROutputInfos that aren't in the
XRRScreenResources we get earlier. This always seems to be transient,
i.e. when it happens, the X server will usually send us a follow up
RRScreenChangeNotify where we then get a "stable" view of the world
again.

In any case, when these glitches happen, we end up with NULL pointers
in the MetaOutput->modes array which makes us crash later on. This
patch ensures that doesn't happen.

https://bugzilla.gnome.org/show_bug.cgi?id=756660