Fix dynamic class zeroing (dotnet#80822)

Recent change in the `ThreadLocalModule::AllocateDynamicClass` has
accidentally removed zeroing for new  CollectibleDynamicEntry instances.
That leads to crashes when running code in an unloadable context.

This change fixes that.

src/coreclr/vm/threadstatics.h

@@ -96,9 +96,9 @@ struct ThreadLocalModule
             LIMITED_METHOD_CONTRACT;
         }
 
-        LOADERHANDLE        m_hGCStatics;
-        LOADERHANDLE        m_hNonGCStatics;
-        PTR_LoaderAllocator m_pLoaderAllocator;
+        LOADERHANDLE        m_hGCStatics = NULL;
+        LOADERHANDLE        m_hNonGCStatics = NULL;
+        PTR_LoaderAllocator m_pLoaderAllocator = NULL;
     };
     typedef DPTR(CollectibleDynamicEntry) PTR_CollectibleDynamicEntry;