DrawTextFormatBar: nonsensical numbers could cause a panic

If one of the numbers provided to draw the progress bar was nonsensical,
like the total download size being 0, the library would cause a panic.

This adds a check for a unexpected current progress, and will display an
empty bar if it's outside acceptable bounds.

draw.go

@@ -108,6 +108,9 @@ func DrawTextFormatBarForW(width int64, w io.Writer) DrawTextFormatFunc {
 
 	return func(progress, total int64) string {
 		current := int64((float64(progress) / float64(total)) * float64(width))
+		if current < 0 || current > width {
+			return fmt.Sprintf("[%s]", strings.Repeat(" ", int(width)))
+		}
 		return fmt.Sprintf(
 			"[%s%s]",
 			strings.Repeat("=", int(current)),

draw_test.go

@@ -44,6 +44,24 @@ func TestDrawTextFormatBar(t *testing.T) {
 	if actual != expected {
 		t.Fatalf("bad: %s", actual)
 	}
+
+	actual = f(0, 0)
+	expected = "[        ]"
+	if actual != expected {
+		t.Fatalf("bad: %s", actual)
+	}
+
+	actual = f(-10, 10)
+	expected = "[        ]"
+	if actual != expected {
+		t.Fatalf("bad: %s", actual)
+	}
+
+	actual = f(20, 10)
+	expected = "[        ]"
+	if actual != expected {
+		t.Fatalf("bad: %s", actual)
+	}
 }
 
 func TestDrawTextFormatBytes(t *testing.T) {