h264: fix overreads in cabac reader.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/cabac_functions.h

@@ -47,6 +47,7 @@ static void refill(CABACContext *c){
         c->low+= c->bytestream[0]<<1;
 #endif
     c->low -= CABAC_MASK;
+    if (c->bytestream < c->bytestream_end)
         c->bytestream += CABAC_BITS / 8;
 }
 
@@ -74,6 +75,7 @@ static void refill2(CABACContext *c){
 #endif
 
     c->low += x<<i;
+    if (c->bytestream < c->bytestream_end)
         c->bytestream += CABAC_BITS/8;
 }