libvulkan: add NULL check to DestroySwapchainKHR

vkDestroySwapchainKHR is required to silently ignore NULL handles
that are provided to it to destroy.
This adds an early return if swapchain_handle is NULL.

Test: dEQP-VK.wsi.android.swapchain.destroy.null_handle (VK 1.0.2 CTS)

Change-Id: Ic230f114a680210cb0e0de931b17e4d70fb27f44