flvmux: unref return of aggregator_pad_peek_buffer

We ended up leaking every single buffer going through the
muxer, which is far from ideal
diff --git a/gst/flv/gstflvmux.c b/gst/flv/gstflvmux.c
index 2efb3d2..68e6572 100644
--- a/gst/flv/gstflvmux.c
+++ b/gst/flv/gstflvmux.c
@@ -1639,6 +1639,7 @@
         best_ts = t;
       }
     }
+    gst_buffer_unref (buffer);
   }
   GST_DEBUG_OBJECT (aggregator,
       "Best pad found with %" GST_TIME_FORMAT ": %" GST_PTR_FORMAT,