We could end up deleting a dirty pointer if the driver set an
output value during a call to vkCreateSwapchainKHR that generates
an error. In practice this turned up in some edge cases with
SwiftShader. Fix this by using a temporary variable.

Bug: chromium:1079475
Change-Id: Ia40e1fac94cbd56fd3bb800a179bb38b7f11fa26
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2216292Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>