Initialise output window semaphore to VK_NULL_HANDLE

In case CreateSemaphore() fails and the semaphore is not valid which could lead to a problem/crash in DestroySemaphore

renderdoc/driver/vulkan/vk_outputwindow.cpp

@@ -943,7 +943,7 @@ void VulkanReplay::BindOutputWindow(uint64_t id, bool depth)
   {
     // semaphore is short lived, so not wrapped, if it's cached (ideally)
     // then it should be wrapped
-    VkSemaphore sem;
+    VkSemaphore sem = VK_NULL_HANDLE;
     VkPipelineStageFlags stage = VK_PIPELINE_STAGE_BOTTOM_OF_PIPE_BIT;
     VkSemaphoreCreateInfo semInfo = {VK_STRUCTURE_TYPE_SEMAPHORE_CREATE_INFO, NULL, 0};