Ensure we don't throw first-chance exceptions if we have a null stack
Ensure we don't throw first-chance exceptions if we have a null stack

This was introduced by 2147918422f18ca598b98fb7e862465db46f674e:
now we may have a null stack on the exception, which it turns out
means new StackTrace(exception).GetFrames() will return null, even
though it's not documented as doing that.

Thankfully this was being swallowed right way, but still not great.

shyamnamboodiripad/roslyn,dotnet/roslyn,jasonmalinowski/roslyn,CyrusNajmabadi/roslyn,bartdesmet/roslyn,shyamnamboodiripad/roslyn,shyamnamboodiripad/roslyn,jasonmalinowski/roslyn,jasonmalinowski/roslyn,mavasani/roslyn,mavasani/roslyn,dotnet/roslyn,dotnet/roslyn,bartdesmet/roslyn,CyrusNajmabadi/roslyn,bartdesmet/roslyn,CyrusNajmabadi/roslyn,mavasani/roslyn