reflect: define MyBuffer more locally in TestImplicitMapConversion
reflect: define MyBuffer more locally in TestImplicitMapConversion

There are no strange messages anymore, so define type MyBuffer in the
block where it is used.

Change-Id: Ic65b15dc76a40f6f734d9ac2116338502fbb66fd
Reviewed-on: https://go-review.googlesource.com/107735
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,cloudflare/go,josharian/go,golang/go,golang/go,golang/go,akutz/go,nawawi/go,golang/go,CAFxX/go,josharian/go,golang/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,nawawi/go,akutz/go,akutz/go