avcodec/av1: Add upper bound for the size of a sane sequence header

It will be used by the Matroska muxer to reserve a certain number
of bytes for the CodecPrivate in case no extradata is initially
available (as it is for the libaom-av1 encoder).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/av1.h

@@ -175,4 +175,10 @@ enum {
     AV1_RESTORE_SWITCHABLE = 3,
 };
 
+// Sequence Headers are actually unbounded because one can use
+// an arbitrary number of leading zeroes when encoding via uvlc.
+// The following estimate is based around using the lowest number
+// of bits for uvlc encoding.
+#define AV1_SANE_SEQUENCE_HEADER_MAX_BITS           3138
+
 #endif /* AVCODEC_AV1_H */