Set mCurrentGraphicsPipeline = nullptr when the pipeline layout is
recreated if immutable samplers are used to prevent a use-after-free.

For a fuller discussion on why this is necessary, see:
Vulkan: clear mCurrentGraphicsPipeline during invalidate |
https://chromium-review.googlesource.com/c/angle/angle/+/2647746

Bug: b/178424566
Bug: angleproject:5624
Change-Id: I70679bda85cd587dbd8893a4576ee5d33aee2b70
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2683041
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>