It's possible for arena_start+MaxArena32 to wrap.
We do the right thing in the bounds check but not in the print.

For #13992 (to fix the print there, not the bug).

Change-Id: I4df845d0c03f0f35461b128e4f6765d3ccb71c6d
Reviewed-on: https://go-review.googlesource.com/18975
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>