avcodec/gif: use the whole allocated buffer

Fixes some gif encoding failures
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/gif.c

@@ -168,7 +168,7 @@ static int gif_image_write_image(AVCodecContext *avctx,
 
     bytestream_put_byte(bytestream, 0x08);
 
-    ff_lzw_encode_init(s->lzw, s->buf, width * height,
+    ff_lzw_encode_init(s->lzw, s->buf, 2 * width * height,
                        12, FF_LZW_GIF, put_bits);
 
     ptr = buf + y_start*linesize + x_start;