i965: Don't try to unmap NULL program cache BO.

i965: Don't try to unmap NULL program cache BO.

When running shader-db with intel_stub and recent Mesa, context creation
fails when making a logical hardware context.  In this case, we call
intelDestroyContext(), which gets here and tries to unmap the cache BO.

But there isn't one - we haven't made it yet.  So we try to unmap a
NULL pointer, which used to be safe (it did nothing), but crashes
after commit 7c3b8ed87859bfdfb985d21685115a729f9cd138.

The result is that we crash rather than failing context creation with
a nice message.  Either way nothing works, but this is more polite.

Cc: "17.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>