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 <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/io/multi.go

@@ -19,6 +19,7 @@ func (mr *multiReader) Read(p []byte) (n int, err error) {
 		}
 		n, err = mr.readers[0].Read(p)
 		if err == EOF {
+			mr.readers[0] = nil // permit earlier GC
 			mr.readers = mr.readers[1:]
 		}
 		if n > 0 || err != EOF {