tiff/lzw: sync (yet again) to the upstream lzw in the stdlib.

This picks up a recent change:
https://go-review.googlesource.com/#/c/11661/

Change-Id: I0309f7b804e0d88ea623c8ae97b65cd0c2bad101
Reviewed-on: https://go-review.googlesource.com/11850
Reviewed-by: David Symonds <dsymonds@golang.org>

tiff/lzw/reader.go

@@ -154,6 +154,7 @@ func (d *decoder) decode() {
 				err = io.ErrUnexpectedEOF
 			}
 			d.err = err
+			d.flush()
 			return
 		}
 		switch {
@@ -205,6 +206,7 @@ func (d *decoder) decode() {
 			}
 		default:
 			d.err = errors.New("lzw: invalid code")
+			d.flush()
 			return
 		}
 		d.last, d.hi = code, d.hi+1