lavf/matroskadec: fix is_keyframe for early Blocks

Blocks are marked as key frames whenever the "reference" field is
zero. This breaks for non-keyframe Blocks with a reference timestamp
of zero.

The likelihood of reference timestamp being zero is increased by a
longstanding bug in muxing that encodes reference timestamp as the
absolute time of the referenced frame (rather than relative to the
current Block timestamp, as described in MKV spec).

Now using INT64_MIN to denote "no reference".

Reported to chromium at http://crbug.com/497889 (contains sample)

libavformat/matroskadec.c

@@ -89,6 +89,7 @@ typedef const struct EbmlSyntax {
     int list_elem_size;
     int data_offset;
     union {
+        int64_t     i;
         uint64_t    u;
         double      f;
         const char *s;
@@ -696,7 +697,7 @@ static const EbmlSyntax matroska_blockgroup[] = {
     { MATROSKA_ID_SIMPLEBLOCK,    EBML_BIN,  0, offsetof(MatroskaBlock, bin) },
     { MATROSKA_ID_BLOCKDURATION,  EBML_UINT, 0, offsetof(MatroskaBlock, duration) },
     { MATROSKA_ID_DISCARDPADDING, EBML_SINT, 0, offsetof(MatroskaBlock, discard_padding) },
-    { MATROSKA_ID_BLOCKREFERENCE, EBML_SINT, 0, offsetof(MatroskaBlock, reference) },
+    { MATROSKA_ID_BLOCKREFERENCE, EBML_SINT, 0, offsetof(MatroskaBlock, reference), { .i = INT64_MIN } },
     { MATROSKA_ID_CODECSTATE,     EBML_NONE },
     {                          1, EBML_UINT, 0, offsetof(MatroskaBlock, non_simple), { .u = 1 } },
     { 0 }
@@ -1071,6 +1072,9 @@ static int ebml_parse_nest(MatroskaDemuxContext *matroska, EbmlSyntax *syntax,
 
     for (i = 0; syntax[i].id; i++)
         switch (syntax[i].type) {
+        case EBML_SINT:
+            *(int64_t *) ((char *) data + syntax[i].data_offset) = syntax[i].def.i;
+            break;
         case EBML_UINT:
             *(uint64_t *) ((char *) data + syntax[i].data_offset) = syntax[i].def.u;
             break;
@@ -3361,7 +3365,7 @@ static int matroska_parse_cluster_incremental(MatroskaDemuxContext *matroska)
         matroska->current_cluster_num_blocks = blocks_list->nb_elem;
         i                                    = blocks_list->nb_elem - 1;
         if (blocks[i].bin.size > 0 && blocks[i].bin.data) {
-            int is_keyframe = blocks[i].non_simple ? !blocks[i].reference : -1;
+            int is_keyframe = blocks[i].non_simple ? blocks[i].reference == INT64_MIN : -1;
             uint8_t* additional = blocks[i].additional.size > 0 ?
                                     blocks[i].additional.data : NULL;
             if (!blocks[i].non_simple)
@@ -3399,7 +3403,7 @@ static int matroska_parse_cluster(MatroskaDemuxContext *matroska)
     blocks      = blocks_list->elem;
     for (i = 0; i < blocks_list->nb_elem; i++)
         if (blocks[i].bin.size > 0 && blocks[i].bin.data) {
-            int is_keyframe = blocks[i].non_simple ? !blocks[i].reference : -1;
+            int is_keyframe = blocks[i].non_simple ? blocks[i].reference == INT64_MIN : -1;
             res = matroska_parse_block(matroska, blocks[i].bin.data,
                                        blocks[i].bin.size, blocks[i].bin.pos,
                                        cluster.timecode, blocks[i].duration,