i915: add a missing NULL pointer check

i915: add a missing NULL pointer check

mesaVisual can be NULL with configless context since this commit:

    commit 551d459af421a2eb937e9e16301bb64da4624f89
    Author: Neil Roberts <neil@linux.intel.com>
    Date:   Fri Mar 7 18:05:47 2014 +0000

    Add the EGL_MESA_configless_context extension
...
    Previously the i965 and i915 drivers were explicitly creating a zeroed visual
    whenever 0 is passed for the EGLConfig.

We attempt to dereference the visual in i915 and now we don't create a
zeroed-out one one it crashes, breaking at least weston in an i915. There's
no point in doing so as it would be zero anyway.

v2: Fixed a typo in commit message.  Added some tags.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1100967
Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>