lzw: check for overread

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

libavcodec/lzw.c

@@ -190,6 +190,10 @@ int ff_lzw_decode(LZWState *p, uint8_t *buf, int len){
             if ((--l) == 0)
                 goto the_end;
         }
+        if (s->ebuf < s->pbuf) {
+            av_log(0, AV_LOG_ERROR, "lzw overread\n");
+            goto the_end;
+        }
         c = lzw_get_code(s);
         if (c == s->end_code) {
             break;