ContextVk::mCurrentGraphicsPipeline should be reset during
ProgramExecutableVk::reset() since programInfo.release() frees the
PipelineHelper that it's pointing to. This has resulted in several
use-after-free errors, which this CL will prevent in the future.

Bug: angleproject:5624
Bug: b/182409935
Change-Id: I847bb7eb5b593c89b84f0fbbca23ea5367f5f55c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2792861
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>