glamor/egl: Avoid crashing on broken configurations

0a9415cf apparently can tickle bugs in the GL stack where glGetString
returns NULL, presumably because the eglMakeCurrent() didn't manage to
actually install a dispatch table and you're hitting a stub function.
That's clearly not our bug, but if it happens we should at least not
crash. Notice this case and fail gently.

Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit af151895)