Prior to this change, broken trailers would be handled by body.Read, and
an error would be returned to its caller (likely a Handler), but that
error would go completely unnoticed by the rest of the server flow
allowing a broken connection to be reused.  This is a possible request
smuggling vector.

Fixes #12027.

Change-Id: I077eb0b8dff35c5d5534ee5f6386127c9954bd58
Reviewed-on: https://go-review.googlesource.com/13148Reviewed-by: Russ Cox <rsc@golang.org>