avcodec/lagarith: Detect end of input in lag_decode_line() loop

Fixes: timeout
Fixes: 2933/clusterfuzz-testcase-5124990208835584

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

libavcodec/lagarith.c

@@ -455,10 +455,12 @@ static int lag_decode_arith_plane(LagarithContext *l, uint8_t *dst,
             return -1;
 
         ff_lag_rac_init(&rac, &gb, length - stride);
-
-        for (i = 0; i < height; i++)
+        for (i = 0; i < height; i++) {
+            if (rac.overread > MAX_OVERREAD)
+                return AVERROR_INVALIDDATA;
             read += lag_decode_line(l, &rac, dst + (i * stride), width,
                                     stride, esc_count);
+        }
 
         if (read > length)
             av_log(l->avctx, AV_LOG_WARNING,

libavcodec/lagarithrac.c

@@ -46,6 +46,7 @@ void ff_lag_rac_init(lag_rac *l, GetBitContext *gb, int length)
     l->range        = 0x80;
     l->low          = *l->bytestream >> 1;
     l->hash_shift   = FFMAX(l->scale, 10) - 10;
+    l->overread     = 0;
 
     for (i = j = 0; i < 1024; i++) {
         unsigned r = i << l->hash_shift;

libavcodec/lagarithrac.h

@@ -47,6 +47,9 @@ typedef struct lag_rac {
     const uint8_t *bytestream;        /**< Current position in input bytestream. */
     const uint8_t *bytestream_end;    /**< End position of input bytestream. */
 
+    int overread;
+#define MAX_OVERREAD 4
+
     uint32_t prob[258];         /**< Table of cumulative probability for each symbol. */
     uint8_t  range_hash[1024];   /**< Hash table mapping upper byte to approximate symbol. */
 } lag_rac;
@@ -62,6 +65,8 @@ static inline void lag_rac_refill(lag_rac *l)
         l->low |= 0xff & (AV_RB16(l->bytestream) >> 1);
         if (l->bytestream < l->bytestream_end)
             l->bytestream++;
+        else
+            l->overread++;
     }
 }