Reject unknown fields in d2i_SSL_SESSION.

The original OpenSSL implementation did the same. M_ASN1_D2I_Finish checks
this. Forwards compatibility with future sessions with unknown fields is
probably not desirable.

Change-Id: I116a8c482cbcc47c3fcc31515c4a3718f66cf268
Reviewed-on: https://boringssl-review.googlesource.com/4941
Reviewed-by: Adam Langley <agl@google.com>