os: use small buffer when reading from windows console

Fixes #5481.

R=golang-dev, dominik.honnef, bradfitz
CC=golang-dev
https://golang.org/cl/9437044

src/pkg/os/file_windows.go

@@ -251,8 +251,14 @@ func (f *File) readConsole(b []byte) (n int, err error) {
 		return 0, nil
 	}
 	if len(f.readbuf) == 0 {
+		// syscall.ReadConsole seems to fail, if given large buffer.
+		// So limit the buffer to 16000 characters.
+		readN := 16000
+		if len(b) < readN {
+			readN = len(b)
+		}
 		// get more input data from os
-		wchars := make([]uint16, len(b))
+		wchars := make([]uint16, readN)
 		var p *uint16
 		if len(b) > 0 {
 			p = &wchars[0]