Be defensive against a null source handle

Author: jkoritzinsky

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Runtime/InteropServices/TrackerObjectManager.NativeAot.cs

@@ -230,12 +230,15 @@ private static unsafe int IFindReferenceTargetsCallback_FoundTrackerTarget(IntPt
                 return HResults.E_INVALIDARG;
             }
 
-            if (!TryGetObject(referenceTrackerTarget, out object? targetObject))
+            if (s_currentRootObjectHandle.Target is not object sourceObject)
             {
                 return HResults.S_FALSE;
             }
 
-            object sourceObject = s_currentRootObjectHandle.Target!;
+            if (!TryGetObject(referenceTrackerTarget, out object? targetObject))
+            {
+                return HResults.S_FALSE;
+            }
 
             if (sourceObject == targetObject)
             {

src/coreclr/vm/interoplibinterface_comwrappers.cpp

@@ -456,6 +456,11 @@ namespace InteropLibImports
         ::OBJECTHANDLE srcHandle = static_cast<::OBJECTHANDLE>(sourceHandle);
         OBJECTREF source = ObjectFromHandle(srcHandle);
 
+        if (source == NULL)
+        {
+            return S_FALSE;
+        }
+
         // Get the target of the external object's reference.
         ::OBJECTHANDLE tgtHandle = static_cast<::OBJECTHANDLE>(targetHandle);
         MOWHOLDERREF holder = (MOWHOLDERREF)ObjectFromHandle(tgtHandle);