parseBase128Int compares |shifted| with four, seemingly to ensure the result
fits in an int32 on 32-bit platforms where int is 32-bit. However, there is an
off-by-one in this logic, so it actually allows five shifts, making the maximum
tag number or OID component 2^35-1.

Fix this so the maximum is 2^28-1 which should be plenty for OID components and
tag numbers while not overflowing on 32-bit platforms.

Change-Id: If825b30cc53a0fc08e68ea1a24d265e7eb1a13a4
Reviewed-on: https://go-review.googlesource.com/18225Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>