avienc: fix AVI stream index for files with >10 streams

Fixes issue 2563.
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/avi.h

@@ -32,6 +32,7 @@
 
 #define AVI_MAX_RIFF_SIZE       0x40000000LL
 #define AVI_MASTER_INDEX_SIZE   256
+#define AVI_MAX_STREAM_COUNT    100
 
 /* index flags */
 #define AVIIF_INDEX             0x10

libavformat/avienc.c

@@ -85,8 +85,8 @@ static int64_t avi_start_new_riff(AVFormatContext *s, AVIOContext *pb,
 
 static char* avi_stream2fourcc(char* tag, int index, enum AVMediaType type)
 {
-    tag[0] = '0';
-    tag[1] = '0' + index;
+    tag[0] = '0' + index/10;
+    tag[1] = '0' + index%10;
     if (type == AVMEDIA_TYPE_VIDEO) {
         tag[2] = 'd';
         tag[3] = 'c';
@@ -158,6 +158,12 @@ static int avi_write_header(AVFormatContext *s)
     int64_t list1, list2, strh, strf;
     AVMetadataTag *t = NULL;
 
+    if (s->nb_streams > AVI_MAX_STREAM_COUNT) {
+        av_log(s, AV_LOG_ERROR, "AVI does not support >%d streams\n",
+               AVI_MAX_STREAM_COUNT);
+        return -1;
+    }
+
     for(n=0;n<s->nb_streams;n++) {
         s->streams[n]->priv_data= av_mallocz(sizeof(AVIStream));
         if(!s->streams[n]->priv_data)