avcodec/dirac_arith: Treat overread as error

Fixes: Timeout
Fixes: 11663/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5636791864918016

Before:Executed clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5636791864918016 in 26006 ms
After: Executed clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5636791864918016 in 106 ms

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dirac_arith.c

@@ -116,6 +116,7 @@ void ff_dirac_init_arith_decoder(DiracArith *c, GetBitContext *gb, int length)
     c->counter = -16;
     c->range   = 0xffff;
     c->error   = 0;
+    c->overread= 0;
 
     for (i = 0; i < DIRAC_CTX_COUNT; i++)
         c->contexts[i] = 0x8000;

libavcodec/dirac_arith.h

@@ -82,6 +82,7 @@ typedef struct {
 
     uint16_t contexts[DIRAC_CTX_COUNT];
     int error;
+    int overread;
 } DiracArith;
 
 extern const uint8_t ff_dirac_next_ctx[DIRAC_CTX_COUNT];
@@ -119,6 +120,9 @@ static inline void refill(DiracArith *c)
                 new |= 0xff00;
 
             c->bytestream = c->bytestream_end;
+            c->overread ++;
+            if (c->overread > 4)
+                c->error = AVERROR_INVALIDDATA;
         }
 
         c->low += new << counter;