avformat/flvdec: do not inject dts=0 metadata packets which failed to be parsed into a new data stream

Such data streams (which then contain no other packets except the faulty one)
confuse some user applications, like VLC
Works around vlcticket 12389

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/flvdec.c

@@ -566,13 +566,13 @@ static int flv_read_metabody(AVFormatContext *s, int64_t next_pos)
     type = avio_r8(ioc);
     if (type != AMF_DATA_TYPE_STRING ||
         amf_get_string(ioc, buffer, sizeof(buffer)) < 0)
-        return -1;
+        return 2;
 
     if (!strcmp(buffer, "onTextData"))
         return 1;
 
     if (strcmp(buffer, "onMetaData") && strcmp(buffer, "onCuePoint"))
-        return -1;
+        return 2;
 
     // find the streams now so that amf_parse_object doesn't need to do
     // the lookup every time it is called.
@@ -830,7 +830,7 @@ static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
             stream_type=FLV_STREAM_TYPE_DATA;
             if (size > 13 + 1 + 4 && dts == 0) { // Header-type metadata stuff
                 meta_pos = avio_tell(s->pb);
-                if (flv_read_metabody(s, next) == 0) {
+                if (flv_read_metabody(s, next) <= 0) {
                     goto skip;
                 }
                 avio_seek(s->pb, meta_pos, SEEK_SET);