ssh/terminal: consume data before checking for an error.

According to the io.Reader docs, Alex had it right the first time. (See
discussion on https://golang.org/cl/25355.)

Change-Id: Ib6fb9dfb99009e034263574e82d7e9d4828df38f
Reviewed-on: https://go-review.googlesource.com/35242
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Adam Langley <agl@golang.org>

ssh/terminal/terminal.go

@@ -930,12 +930,6 @@ func readPasswordLine(reader io.Reader) ([]byte, error) {
 
 	for {
 		n, err := reader.Read(buf[:])
-		if err != nil {
-			if err == io.EOF && len(ret) > 0 {
-				return ret, nil
-			}
-			return ret, err
-		}
 		if n > 0 {
 			switch buf[0] {
 			case '\n':
@@ -945,6 +939,13 @@ func readPasswordLine(reader io.Reader) ([]byte, error) {
 			default:
 				ret = append(ret, buf[0])
 			}
+			continue
+		}
+		if err != nil {
+			if err == io.EOF && len(ret) > 0 {
+				return ret, nil
+			}
+			return ret, err
 		}
 	}
 }