open modified the plugin symbols map while ranging over it. This is
normally harmless, except that the operations performed were not
idempotent leading to function pointers being corrupted.

Fixes #19269

Change-Id: I4b6eb1d45567161412e4a34b41f1ebf647bcc942
Reviewed-on: https://go-review.googlesource.com/40431
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>