avformat/mxfenc: Simplfy writing padding

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

libavformat/mxfenc.c

@@ -965,7 +965,6 @@ static void mxf_write_structural_component(AVFormatContext *s, AVStream *st, MXF
 {
     MXFContext *mxf = s->priv_data;
     AVIOContext *pb = s->pb;
-    int i;
 
     mxf_write_metadata_key(pb, 0x011100);
     PRINT_KEY(s, "sturctural component key", pb->buf_ptr - 16);
@@ -985,8 +984,7 @@ static void mxf_write_structural_component(AVFormatContext *s, AVStream *st, MXF
     // write source package uid, end of the reference
     mxf_write_local_tag(s, 32, 0x1101);
     if (!package->ref) {
-        for (i = 0; i < 4; i++)
+        ffio_fill(pb, 0, 32);
-            avio_wb64(pb, 0);
     } else
         mxf_write_umid(s, package->ref->instance);