avcodec/vp56rac: delay signaling an error on truncated input

A threshold of 1 is sufficient for simple_dump_cut.webm, 10 is used
just to be sure the next truncated file doesnt cause the same issue

Obvious alternative fixes are to simply accept that the file is broken or to
write some advanced error concealment or to
simply accept that the decoder wont stop at the end of input.

Fixes: Ticket 8069 (artifacts not the differing md5 which was there before 1afd246960)
Fixes: simple_dump_cut.webm
Fixes: regression of 1afd246960

fate-vp5 changes because the last frame is truncated and now handled
differently.

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

libavcodec/vp56.h

@@ -89,6 +89,7 @@ typedef struct VP56RangeCoder {
     const uint8_t *buffer;
     const uint8_t *end;
     unsigned int code_word;
+    int end_reached;
 } VP56RangeCoder;
 
 typedef struct VP56RefDc {
@@ -235,7 +236,9 @@ int ff_vp56_init_range_decoder(VP56RangeCoder *c, const uint8_t *buf, int buf_si
  */
 static av_always_inline int vpX_rac_is_end(VP56RangeCoder *c)
 {
-    return c->end <= c->buffer && c->bits >= 0;
+    if (c->end <= c->buffer && c->bits >= 0)
+        c->end_reached ++;
+    return c->end_reached > 10;
 }
 
 static av_always_inline unsigned int vp56_rac_renorm(VP56RangeCoder *c)

libavcodec/vp56rac.c

@@ -43,6 +43,7 @@ int ff_vp56_init_range_decoder(VP56RangeCoder *c, const uint8_t *buf, int buf_si
     c->bits = -16;
     c->buffer = buf;
     c->end = buf + buf_size;
+    c->end_reached = 0;
     if (buf_size < 1)
         return AVERROR_INVALIDDATA;
     c->code_word = bytestream_get_be24(&c->buffer);

tests/ref/fate/vp5

@@ -249,4 +249,4 @@
 0,        243,        243,        1,   233472, 0x6f530ac6
 0,        244,        244,        1,   233472, 0x94f7466c
 0,        245,        245,        1,   233472, 0xa8c1d365
-0,        246,        246,        1,   233472, 0xbf73f1b7
+0,        246,        246,        1,   233472, 0x4f3ef38c