runtime: fix finalizer iterator

It could only handle one finalizer before it raised an out-of-bounds error.

Fixes issue #9172

Change-Id: Ibb4d0c8aff2d78a1396e248c7129a631176ab427
Reviewed-on: https://go-review.googlesource.com/1201
Reviewed-by: Russ Cox <rsc@golang.org>