Fix switching between AEAD and non-AEAD in a renegotiation.

https://code.google.com/p/chromium/issues/detail?id=423998

Change-Id: I29d67db92b47d6cd303125b44e5ba552d97d54ff
Reviewed-on: https://boringssl-review.googlesource.com/1960
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>