Crashes with NULL XVisualInfo

Original bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=841951

The problem is basically that src/platformsupport/glxconvenience/qglxconvenience.cpp::208 is not checked for NULL. It seems our user has an issue with his video card/software stack.

Anthony Towns created a simple test case for the user to reproduce:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=841951#32 and https://bugs.debian.org/cgi-bin/bugreport.cgi?att=1;bug=841951;filename=test.cpp;msg=32

which returns NULL in one case:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=841951#58

So I don't know for sure where is the real bug, but I think this line should be checked for NULL at least to avoid the crash. Before submitting the one-liner that fixes it I would like to know if you want some more info to debug this.