avformat:matroskadec: use a define to mark the EBML length is unknown

Unifying the way the EBML unknown length is signaled, rather than using two
incompatible values. UINT64_MAX cannot be read as a valid EBML length with the
current code.
Co-authored-by: Steve Lhomme <robux4@ycbcr.xyz>
Co-authored-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -68,6 +68,8 @@
 
 #include "qtpalette.h"
 
+#define EBML_UNKNOWN_LENGTH  UINT64_MAX /* EBML unknown length, in uint64_t */
+
 typedef enum {
     EBML_NONE,
     EBML_UINT,
@@ -869,7 +871,7 @@ static int ebml_read_length(MatroskaDemuxContext *matroska, AVIOContext *pb,
 {
     int res = ebml_read_num(matroska, pb, 8, number);
     if (res > 0 && *number + 1 == 1ULL << (7 * res))
-        *number = 0xffffffffffffffULL;
+        *number = EBML_UNKNOWN_LENGTH;
     return res;
 }
 
@@ -1049,7 +1051,7 @@ static int ebml_parse_id(MatroskaDemuxContext *matroska, EbmlSyntax *syntax,
             break;
     if (!syntax[i].id && id == MATROSKA_ID_CLUSTER &&
         matroska->num_levels > 0                   &&
-        matroska->levels[matroska->num_levels - 1].length == 0xffffffffffffff)
+        matroska->levels[matroska->num_levels - 1].length == EBML_UNKNOWN_LENGTH)
         return 0;  // we reached the end of an unknown size cluster
     if (!syntax[i].id && id != EBML_ID_VOID && id != EBML_ID_CRC32) {
         av_log(matroska->ctx, AV_LOG_DEBUG, "Unknown entry 0x%"PRIX32"\n", id);
@@ -1201,7 +1203,7 @@ static int ebml_parse_elem(MatroskaDemuxContext *matroska,
             MatroskaLevel *level = &matroska->levels[matroska->num_levels - 1];
             AVIOContext *pb = matroska->ctx->pb;
             int64_t pos = avio_tell(pb);
-            if (level->length != (uint64_t) -1 &&
+            if (level->length != EBML_UNKNOWN_LENGTH &&
                 (pos + length) > (level->start + level->length)) {
                 av_log(matroska->ctx, AV_LOG_ERROR,
                        "Invalid length 0x%"PRIx64" > 0x%"PRIx64" in parent\n",
@@ -1610,7 +1612,7 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
             ret = AVERROR_INVALIDDATA;
         } else {
             level.start  = 0;
-            level.length = (uint64_t) -1;
+            level.length = EBML_UNKNOWN_LENGTH;
             matroska->levels[matroska->num_levels] = level;
             matroska->num_levels++;
             matroska->current_id                   = 0;
@@ -1620,7 +1622,7 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
             /* remove dummy level */
             while (matroska->num_levels) {
                 uint64_t length = matroska->levels[--matroska->num_levels].length;
-                if (length == (uint64_t) -1)
+                if (length == EBML_UNKNOWN_LENGTH)
                     break;
             }
         }