avformat/matroskadec: Fix default value of BlockAddID

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -707,7 +707,7 @@ static EbmlSyntax matroska_segments[] = {
 };
 
 static EbmlSyntax matroska_blockmore[] = {
-    { MATROSKA_ID_BLOCKADDID,      EBML_UINT, 0, offsetof(MatroskaBlock,additional_id) },
+    { MATROSKA_ID_BLOCKADDID,      EBML_UINT, 0, offsetof(MatroskaBlock,additional_id), { .u = 1 } },
     { MATROSKA_ID_BLOCKADDITIONAL, EBML_BIN,  0, offsetof(MatroskaBlock,additional) },
     CHILD_OF(matroska_blockadditions)
 };