Check for null Pointer in _cogl_gl_util_parse_gl_version

This part of the code crashed gnome-session on some systems when running it with libcogl.so.12.1.1 on Fedora 19.
Those systems have a "NVIDIA Corporation GF106 [GeForce GTS 450] (rev a1)" graphics card with the proprietary driver.
Somehow the version string is a null pointer with those..