io: make MultiReader nil exhausted Readers for earlier GC
io: make MultiReader nil exhausted Readers for earlier GC

No test because the language spec makes no promises in this area.

Fixes #16983

Change-Id: I1a6aa7ff87dd14aa27e8400040a6f6fc908aa1fd
Reviewed-on: https://go-review.googlesource.com/28533
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

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