PKCS#1 v2.1 section 7.1.1 says that the result of an OAEP encryption
is "an octet string of length $k$". Since we didn't left-pad the
result it was previously possible for the result to be smaller when
the most-significant byte was zero.

Fixes #1519.

R=rsc
CC=golang-dev
https://golang.org/cl/4175059