Fix Utf8JsonWriter.Dispose{Async} after failed Stream Write{Async} (dotnet/corefx...

Fix Utf8JsonWriter.Dispose{Async} after failed Stream Write{Async} (dotnet/corefx#39836)

* Fix Utf8JsonWriter.Dispose{Async} after failed Stream Write{Async}

By the time the `Utf8JsonWriter` writes to the underlying `Stream` as part of `Flush{Async}` it has already `Advance`'d the `_arrayBufferWriter`, but it hasn't reset `BytesPending` to 0.  If the write to the stream throws an exception then, we end up in an inconsistent state where `BytesPending` != 0 but there's no corresponding pending bytes to `Advance` the `_arrayBufferWriter`.  When we then `Dispose` the `Utf8JsonWriter`, we attempt to `Advance` the `_arrayBufferWriter` and fail.  Since `Utf8JsonWriter` will often be used in a `using` block, an exception from a `Write`/`Flush{Async}` call inside the `using` will then be masked as the `using`'s call to `Dispose` ends up failing and throwing a secondary exception.

* Address PR feedback

* Address PR feedback

Commit migrated from https://github.com/dotnet/corefx/commit/c38f2327e2d4e159acba06e23f38daed24661652