avformat/matroskadec: Fix default value of BlockAddID

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit dbc50f8a935043243232b2e01f3c012ab6d49928)
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 1d83068..8a392c7 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -660,7 +660,7 @@
 };
 
 static const 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) },
     { 0 }
 };