encoding/unicode: fix UTF-16/32 decoding of an empty src.

Change-Id: I9b94e32745364ebb234904ca4e4c51ec7021d898
Reviewed-on: https://go-review.googlesource.com/23931
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
4 files changed