i965: Avoid segfault in gen6_upload_state

i965: Avoid segfault in gen6_upload_state

This fixes a bug introduced in commit 258453716f001eab1288d99765213 and
triggered whenever "rb" is NULL.

Fixes at least one cause bug #59445:

	[SNB/IVB/HSW Bisected]Oglc draw-buffers2(advanced.blending.none) segfault
	https://bugs.freedesktop.org/show_bug.cgi?id=59445

(Though segfaults are still possible in that test case, but they have been
present since before commit 258453716f which is what's being fixed here.)

Reviewed-by: Eric Anholt <eric@anholt.net>