avcodec/rangecoder: Do not increase the pointer beyond the buffer

Fixes: undefined behavior
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/rangecoder.c

@@ -58,6 +58,7 @@ av_cold void ff_init_range_decoder(RangeCoder *c, const uint8_t *buf,
 
     c->low         = AV_RB16(c->bytestream);
     c->bytestream += 2;
+    c->overread    = 0;
     if (c->low >= 0xFF00) {
         c->low = 0xFF00;
         c->bytestream_end = c->bytestream;

libavcodec/rangecoder.h

@@ -42,6 +42,8 @@ typedef struct RangeCoder {
     uint8_t *bytestream_start;
     uint8_t *bytestream;
     uint8_t *bytestream_end;
+    int overread;
+#define MAX_OVERREAD 2
 } RangeCoder;
 
 void ff_init_range_encoder(RangeCoder *c, uint8_t *buf, int buf_size);
@@ -106,9 +108,11 @@ static inline void refill(RangeCoder *c)
     if (c->range < 0x100) {
         c->range <<= 8;
         c->low   <<= 8;
-        if (c->bytestream < c->bytestream_end)
+        if (c->bytestream < c->bytestream_end) {
             c->low += c->bytestream[0];
             c->bytestream++;
+        } else
+            c->overread ++;
     }
 }