Fix Decode to return the correct illegal data index from a corrupted
input that contains whitespaces.

Fixes #19406

Change-Id: Ib2b2b6ed7e41f024d0da2bd035caec4317c2869c
Reviewed-on: https://go-review.googlesource.com/37837Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>