Vulkan: Initialize all members of VmaVulkanFunctions. VmaVulkanFunctions was passed to vmaCreateAllocator without initializing all members. This leads to potential reads of uninitialized variables. BUG=chromium:1078652 Change-Id: Id349c8fbb2dd3a5a2c64c6a017add3d6f97b78fd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2185191 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>