The use in s3_srvr.c doesn't care (it doesn't even have to be in bounds), but
it's good to have the value be initialized and not a function of the input.
(The old uninitialized case wasn't hit in s3_srvr.c because of the earlier
bounds check.)

Change-Id: Ib6b418b3c140aa564f8a46da3d34bb2b69f06195
Reviewed-on: https://boringssl-review.googlesource.com/2845
Reviewed-by: Adam Langley <agl@google.com>