Bitwise-or, not logical-or, when hashing.

This didn't actually break anything, but it does make session lookup
quite slow.

Change-Id: I13615e8ccf6a46683a21774eb7c073318ae8c28c
Reviewed-on: https://boringssl-review.googlesource.com/6054
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: Adam Langley <agl@google.com>