term: consistently return zeroes on GetSize error

All other platform have return value of 0 for error when trying to get
the terminal size while unix have -1. That makes it hard to catch
whether the value passed from it was inserted on purpose.

Change-Id: If47d07bd2d4f6cd439ad0ff4958e4095c8e4df9d
GitHub-Last-Rev: 39cb14d
GitHub-Pull-Request: #6
Reviewed-on: https://go-review.googlesource.com/c/term/+/506235
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

term_unix.go

@@ -60,7 +60,7 @@ func restore(fd int, state *State) error {
 func getSize(fd int) (width, height int, err error) {
 	ws, err := unix.IoctlGetWinsize(fd, unix.TIOCGWINSZ)
 	if err != nil {
-		return -1, -1, err
+		return 0, 0, err
 	}
 	return int(ws.Col), int(ws.Row), nil
 }