Fix for issue 6768 NullRef when access Level when there are EventSource messages.

Fix for issue 6768 NullRef when access Level when there are EventSource messages.

See https://github.com/dotnet/coreclr/issues/6768 for mor details on the bug.

The issue is that EventSource is firing events (to report errors) before the EventSource
is completely initialized.   We dealt with many of the issues associated with this but
not all of them.   This change makes these special eventSource events (with ID of 0
 take the same codepath as for TraceLogging events (with negative IDs) when the
EventWrittenArgs properies are accessed.

Commit migrated from https://github.com/dotnet/coreclr/commit/c728f1faf06b5d77bd185f81939c55c1c0358ede