jv demuxer: define JV_PREAMBLE_SIZE instead of hard coding the number

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/jvdec.c

@@ -28,6 +28,8 @@
 #include "libavutil/intreadwrite.h"
 #include "avformat.h"
 
+#define JV_PREAMBLE_SIZE 5
+
 typedef struct {
     int audio_size;    /** audio packet size (bytes) */
     int video_size;    /** video packet size (bytes) */
@@ -154,15 +156,15 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
             jv->state++;
             if (jvf->video_size || jvf->palette_size) {
                 int size = jvf->video_size + jvf->palette_size;
-                if (av_new_packet(pkt, size + 5))
+                if (av_new_packet(pkt, size + JV_PREAMBLE_SIZE))
                     return AVERROR(ENOMEM);
 
                 AV_WL32(pkt->data, jvf->video_size);
                 pkt->data[4]      = jvf->video_type;
-                if (avio_read(pb, pkt->data + 5, size) < 0)
+                if (avio_read(pb, pkt->data + JV_PREAMBLE_SIZE, size) < 0)
                     return AVERROR(EIO);
 
-                pkt->size         = size + 5;
+                pkt->size         = size + JV_PREAMBLE_SIZE;
                 pkt->stream_index = 1;
                 pkt->pts          = jv->pts;
                 if (jvf->video_type != 1)