fmp4mux: Only push fragment_offset if write_mfra is true

This is done so that the fragment_offset vector does not infinitely
build up when write_mfra is disabled.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1023>

mux/fmp4/src/fmp4mux/imp.rs

@@ -1505,6 +1505,7 @@ impl FMP4Mux {
                     .collect::<gst::BufferList>(),
             );
 
+            if settings.write_mfra {
                 // Write mfra only for the main stream, and if there are no buffers for the main stream
                 // in this segment then don't write anything.
                 if let Some(super::FragmentHeaderStream {
@@ -1517,6 +1518,7 @@ impl FMP4Mux {
                         offset: moof_offset,
                     });
                 }
+            }
 
             state.end_pts = Some(fragment_end_pts);