Disallow all special operators once groups are used.

+ and - should also be forbidden. Any operation other than appending will mix
up the in_group bits and give unexpected behavior.

Change-Id: Ieaebb9ee6393aa36243d0765e45cae667f977ef5
Reviewed-on: https://boringssl-review.googlesource.com/1803
Reviewed-by: Adam Langley <agl@google.com>