avformat/flvenc: flx flvflags no_metadata bug

When use flvflags no_metadata , the FLV header will be cover by write tailer
This commit fix the bug
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/flvenc.c

@@ -848,6 +848,7 @@ end:
         avio_seek(pb, flv->datasize_offset, SEEK_SET);
         put_amf_double(pb, flv->datasize);
     }
+    if (!(flv->flags & FLV_NO_METADATA)) {
         if (!(flv->flags & FLV_NO_DURATION_FILESIZE)) {
             /* update information */
             if (avio_seek(pb, flv->duration_offset, SEEK_SET) < 0) {
@@ -861,7 +862,8 @@ end:
                 put_amf_double(pb, file_size);
             }
         }
-    avio_seek(pb, file_size, SEEK_SET);
+    }
+
     return 0;
 }