cmd/compile/internal/syntax: avoid deadlock

cmd/compile/internal/syntax: avoid deadlock

When the err from ReadFile is non-nil, we call t.Fatal(err).
Switch t.Fatal to t.Error + return.
ensure that close(results) happens on that code path as well.

Updates #17697.

Change-Id: Ifaacf27a76c175446d642086ff32f4386428080d
Reviewed-on: https://go-review.googlesource.com/32486
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>