Update for renamed aggregator pad API

https://bugzilla.gnome.org/show_bug.cgi?id=791204
diff --git a/gst/flv/gstflvmux.c b/gst/flv/gstflvmux.c
index a4e151d..ac907e0 100644
--- a/gst/flv/gstflvmux.c
+++ b/gst/flv/gstflvmux.c
@@ -1616,7 +1616,7 @@
   for (l = GST_ELEMENT_CAST (aggregator)->sinkpads; l; l = l->next) {
     apad = GST_AGGREGATOR_PAD (l->data);
     pad = GST_FLV_MUX_PAD (l->data);
-    buffer = gst_aggregator_pad_get_buffer (GST_AGGREGATOR_PAD (pad));
+    buffer = gst_aggregator_pad_peek_buffer (GST_AGGREGATOR_PAD (pad));
     if (!buffer)
       continue;
     if (best_ts == GST_CLOCK_TIME_NONE) {
@@ -1679,7 +1679,7 @@
   }
 
   if (best) {
-    buffer = gst_aggregator_pad_steal_buffer (GST_AGGREGATOR_PAD (best));
+    buffer = gst_aggregator_pad_pop_buffer (GST_AGGREGATOR_PAD (best));
     g_assert (buffer);
     best->dts =
         gst_flv_mux_segment_to_running_time (&GST_AGGREGATOR_PAD