MPIs are (supposed to be) stripped of leading zeroes. Avoid passing
such short values into crypto/rsa, even if it currently happens to work.
Change-Id: I5a5f4813b8358e83fcc2deeda1272d2733814542
Reviewed-on: https://go-review.googlesource.com/100844
Reviewed-by: Adam Langley <agl@golang.org>