Fixes #3982

When splitting buffers in a stream, the node stores any 'left-over' bits following the split to be joined with the next message in the stream. It stored the done event until that left-over bit was made whole.

However, if there were no left-over bits (length === 0), it was storing done for later and calling done.

The fix is to not store the done for later if there is no left-over bit.

But, this does also raise a question of a difference in behaviour between string and buffer modes. I think for String streams, it always calls done after splitting, regardless of any left-over bits... whereas for Buffer streams, as described above, it waits until left-overs are handled.

This could do with tidying up, but needs more investigation and test cases to properly figure out the 'expected' behaviour.