openpgp: Fix panic on v3 public keys with too-short modulus.

Found using gofuzz.

Fixes golang/go#11504

Change-Id: I49cf01e75e37c5d87dad58c5349161d79d0b72f5
Reviewed-on: https://go-review.googlesource.com/12635
Reviewed-by: Adam Langley <agl@golang.org>
2 files changed