Base64-encoding 32 bytes results in a 44-byte string.
While in general a 44-byte string might decode to 33 bytes,
if you take a 44-byte string that actually only encodes 32 bytes,
and you try to decode it into 32 bytes, that should succeed.
Instead it fails trying to do a useless dst[33:] slice operation.
Delete that slice operation.

Noticed while preparing CL 156322.

Change-Id: I8024bf28a65e2638675b980732b2ff91c66c62cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/164628Reviewed-by: Ian Lance Taylor <iant@golang.org>