lzw(gif): Fix overread

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/lzw.c

@@ -101,7 +101,7 @@ void ff_lzw_decode_tail(LZWState *p)
     struct LZWState *s = (struct LZWState *)p;
 
     if(s->mode == FF_LZW_GIF) {
-        while(s->pbuf < s->ebuf && s->bs>0){
+        while(s->pbuf + s->bs < s->ebuf && s->bs>0){
             s->pbuf += s->bs;
             s->bs = *s->pbuf++;
         }