Initialize pointer members in VulkanManager to null

Test: manual local testing

Change-Id: If81db17858354844b96677137cf67449dd91854b
diff --git a/libs/hwui/renderthread/VulkanManager.cpp b/libs/hwui/renderthread/VulkanManager.cpp
index 1d8cdd6..a693e68 100644
--- a/libs/hwui/renderthread/VulkanManager.cpp
+++ b/libs/hwui/renderthread/VulkanManager.cpp
@@ -602,6 +602,7 @@
         mDeviceWaitIdle(mBackendContext->fDevice);
     }
 
+    SkASSERT(surface->mBackbuffers);
     VulkanSurface::BackbufferInfo* backbuffer =
             surface->mBackbuffers + surface->mCurrentBackbufferIndex;
     GrVkImageInfo* imageInfo;
@@ -683,6 +684,7 @@
 }
 
 int VulkanManager::getAge(VulkanSurface* surface) {
+    SkASSERT(surface->mBackbuffers);
     VulkanSurface::BackbufferInfo* backbuffer =
             surface->mBackbuffers + surface->mCurrentBackbufferIndex;
     if (mSwapBehavior == SwapBehavior::Discard ||