cmd/cover: fix off-by-one error in TestCoverHTML

Avoid index out of range if len(goldenLines) == len(outLines) + 1

Change-Id: Ic23a85d2b8dd06a615e35a58331e78abe4ad6703
Reviewed-on: https://go-review.googlesource.com/130396
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cover/cover_test.go

@@ -314,7 +314,7 @@ func TestCoverHTML(t *testing.T) {
 	// Compare at the line level, stopping at first different line so
 	// we don't generate tons of output if there's an inserted or deleted line.
 	for i, goldenLine := range goldenLines {
-		if i > len(outLines) {
+		if i >= len(outLines) {
 			t.Fatalf("output shorter than golden; stops before line %d: %s\n", i+1, goldenLine)
 		}
 		// Convert all white space to simple spaces, for easy comparison.