xcb plugin SIGFPEs when modeInfo's htotal and/or vtotal are 0

This might be a corner case, but I still think it's valid.

When we in the Debian Qt/KDE team build our packages we try to run their tests. In this case Maxy Curia was running kidentitymanagement's tests under xvfb (because the build farms do not run X).

Maxy discovered that qxcbscreen.cpp::547 [0] assumes that both htotal and vtotal are >0, but that's not the case with xvfb.

I would simply check they are not 0 and then use some default value for m_refreshRate. Of course if you tell me how to proceed I'll happily provide a patch through gerrit.

Thanks in advance!

[0] <http://sources.debian.net/src/qtbase-opensource-src/5.5.1%2Bdfsg-13/src/plugins/platforms/xcb/qxcbscreen.cpp/?hl=547#L547>