Return error when a bit string indicates an invalid amount of bits left

(Imported from upstream's 5a1e8c67a90aead86ccc2dda324e8f897d1a044d)

Change-Id: Idfba7eb8244c1926e7921119767cb32605a74202
Reviewed-on: https://boringssl-review.googlesource.com/2836
Reviewed-by: Adam Langley <agl@google.com>