Without this, each additional C frame found via SetCgoTraceback will
cause a frame to be dropped from the bottom of the traceback stack.

Fixes #29034

Change-Id: I90aa6b2a1dced90c69b64c5dd565fe64a25724a3
Reviewed-on: https://go-review.googlesource.com/c/151917
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>