compress/lzw: silently drop implied codes that are too large,

instead of returning an error.

For example, http://www.w3.org/Graphics/GIF/spec-gif89a.txt
explicitly says that GIF encoders can use a full table as is,
without needing to send a clear code.

R=r, dsymonds, nigeltao_gnome, r2
CC=golang-dev
https://golang.org/cl/4518041

src/pkg/compress/lzw/reader.go

@@ -165,16 +165,19 @@ func decode1(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os
 			if _, err := w.Write(buf[i:]); err != nil {
 				return err
 			}
+			if last != invalidCode {
 				// Save what the hi code expands to.
 				suffix[hi] = uint8(c)
 				prefix[hi] = last
+			}
 		default:
 			return os.NewError("lzw: invalid code")
 		}
 		last, hi = code, hi+1
-		if hi == overflow {
+		if hi >= overflow {
 			if d.width == maxWidth {
-				return os.NewError("lzw: missing clear code")
+				last = invalidCode
+				continue
 			}
 			d.width++
 			overflow <<= 1