Fix protocol downgrade bug in case of fragmented packets

CVE-2014-3511

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Bodo Möller <bodo@openssl.org>

(Imported from upstream's 280b1f1ad12131defcd986676a8fc9717aaa601b)

Change-Id: I39894b340d2a03751b221631f8cd0ee220cf5b67
Reviewed-on: https://boringssl-review.googlesource.com/1451
Reviewed-by: Adam Langley <agl@google.com>