D3D12: check if resource manager exists before releasing

ClusterFuzz injects errors into the device which may not
initialize the resource manager which would later fail upon
shutdown. This adds a check to see if the resource manager
exists before releasing the pool.

BUG=chromium:1117595

Change-Id: Ie2b90f947f3ddd3906ecabc65e959915c1f7e386
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/27040
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

src/dawn_native/d3d12/DeviceD3D12.cpp

@@ -571,7 +571,9 @@ namespace dawn_native { namespace d3d12 {
         }
 
         // Release recycled resource heaps.
+        if (mResourceAllocatorManager != nullptr) {
             mResourceAllocatorManager->DestroyPool();
+        }
 
         // We need to handle clearing up com object refs that were enqeued after TickImpl
         mUsedComObjectRefs.ClearUpTo(std::numeric_limits<Serial>::max());