Fix the derivation of SSL3_RT_SEND_MAX_ENCRYPTED_OVERHEAD.

It happens to give the same value anyway (64 + 16), but only on accident.

Change-Id: I1415f4015e3de472dbeb9ada0d92607c9d1bcd40
Reviewed-on: https://boringssl-review.googlesource.com/3780
Reviewed-by: Adam Langley <agl@google.com>