This never happens but for pathological input where a BOM sequence
is unfinished and ends in EOF (src: "package p\n\nfunc \xef\xef").
No test case added because the /test framework doesn't lend itself
easily to it in this case (file must end in EOF rather than comment).
Instead, tested manually.

Fixes #13268.

Change-Id: I049034e6dde7ad884b0a8c329921adac1866ff18
Reviewed-on: https://go-review.googlesource.com/17047Reviewed-by: Chris Manghane <cmang@golang.org>