io: fix test when MultiReader is inlined with -l=3
io: fix test when MultiReader is inlined with -l=3

This ensures there isn't a live reference to buf1 on our stack
when MultiReader is inlined.

Fixes #18819.

Change-Id: I96a8cdc1ffad8f8a10c0ddcbf0299005f3176b61
Reviewed-on: https://go-review.googlesource.com/35931
Run-TryBot: David Lazar <859c8a6d485c34c743019c2e216e6c66f2ebb470@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

codestation/go,ImJasonH/go,akutz/go,Cofyc/go,polyverse-security/go,Cofyc/go,Cofyc/go,nawawi/go,vsekhar/elastic-go,cloudflare/go,CAFxX/go,nawawi/go,linux-on-ibm-z/go,derekmarcotte/go,golang/go,polyverse-security/go,CAFxX/go,ImJasonH/go,golang/go,codestation/go,CAFxX/go,derekmarcotte/go,nawawi/go,akutz/go,josharian/go,golang/go,akutz/go,cloudflare/go,akutz/go,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,nawawi/go,derekmarcotte/go,linux-on-ibm-z/go,Cofyc/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,CAFxX/go,golang/go,polyverse-security/go,vsekhar/elastic-go,christopher-henderson/Go,akutz/go,cloudflare/go,ImJasonH/go,codestation/go,vsekhar/elastic-go,codestation/go,nawawi/go,linux-on-ibm-z/go,cloudflare/go,codestation/go,cloudflare/go,Cofyc/go,josharian/go,vsekhar/elastic-go,josharian/go,polyverse-security/go,cloudflare/go,vsekhar/elastic-go,derekmarcotte/go,golang/go,CAFxX/go,ImJasonH/go,golang/go,CAFxX/go,codestation/go,golang/go,akutz/go,ImJasonH/go,josharian/go,codestation/go,cloudflare/go,golang/go,linux-on-ibm-z/go,josharian/go,linux-on-ibm-z/go,nawawi/go,linux-on-ibm-z/go,derekmarcotte/go,Cofyc/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,ImJasonH/go,akutz/go,cloudflare/go,Cofyc/go,polyverse-security/go,CAFxX/go,nawawi/go,josharian/go,polyverse-security/go,josharian/go,linux-on-ibm-z/go,derekmarcotte/go,ImJasonH/go,nawawi/go,ImJasonH/go