mp4: set amr-nb samplerate to 8000, as it's only valid one

Fixes some 3gp-videos with amr-nb audio.

modules/demux/mp4/mp4.c

@@ -1895,9 +1895,10 @@ static int TrackCreateES( demux_t *p_demux, mp4_track_t *p_track,
                             p_track->fmt.i_extra);
                 }
                 break;
+            case VLC_FOURCC( 's', 'a', 'm', 'r' ):
+                p_track->fmt.audio.i_rate = 8000;
             case VLC_FOURCC( 'Q', 'D', 'M', 'C' ):
             case VLC_FOURCC( 'Q', 'D', 'M', '2' ):
-            case VLC_FOURCC( 's', 'a', 'm', 'r' ):
             case VLC_FOURCC( 'a', 'l', 'a', 'c' ):
                 p_track->fmt.i_extra =
                     p_sample->data.p_sample_soun->i_qt_description;