Destroying ISymUnmanagedReader in SymReader Dispose plus calling dispose

This bug is not easy to reproduce but ever since I added this I can't reproduce but I can't be 100% if its luck or fix... If someone has good way of reproducing please confirm this fix...

I don't think this can bring any regression...

From what I checked new SymReader is created only in two places 1 is OnUpdateModuleSymbols where I now instantly dispose and other is in OnModuleLoad where is then put in list of modules and documents where is disposed in CorSessionDebugger dispose.