[vtkusers] gl_vendor on Linux missing crash the app

[Dženan Zukić]

I was testing my program under Linux (Fedora 14), and it crashes in
vtkOpenGLGPUVolumeRayCastMapper.cxx on line 2080. The reason is that
call to glGetString(GL_VENDOR) on previous line returns NULL, which in
turn crashes strstr(). I am using nVidia proprietary driver (installed
some months ago, probably not the latest version).

Is this a known problem?