image/gif: close the lzw.Reader we create.

The lzw.NewReader doc comment says, "It is the caller's responsibility
to call Close on the ReadCloser when finished reading."

Thanks to Andrew Bonventre for noticing this.

R=r, dsymonds, adg
CC=andybons, golang-dev
https://golang.org/cl/10821043

src/pkg/image/gif/reader.go

@@ -190,6 +190,7 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 			// A wonderfully Go-like piece of magic.
 			br := &blockReader{r: d.r}
 			lzwr := lzw.NewReader(br, lzw.LSB, int(litWidth))
+			defer lzwr.Close()
 			if _, err = io.ReadFull(lzwr, m.Pix); err != nil {
 				if err != io.ErrUnexpectedEOF {
 					return err