If an encrypted PEM block contained ciphertext that was not a multiple
of the block size then the code would panic. This change tests for that
case and returns an error.

Fixes #11215.

Change-Id: I7b700f99e20810c4f545519b1e9d766b4640e8a7
Reviewed-on: https://go-review.googlesource.com/11097Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>