plugin: resolve random crash when calling exported functions

plugin: resolve random crash when calling exported functions

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>