inputselector: Fix buffer leak in sync_streams & cache_buffers mode

After doing gst_pad_push() in case of sync_streams and cache_buffers,
if the buffer can not be kept in cache, it should be unreffed to avoid
memory leackage.

https://bugzilla.gnome.org/show_bug.cgi?id=755141

plugins/elements/gstinputselector.c

@@ -376,6 +376,7 @@ gst_selector_pad_cache_buffer (GstSelectorPad * selpad, GstBuffer * buffer)
   if (selpad->segment.format != GST_FORMAT_TIME) {
     GST_DEBUG_OBJECT (selpad, "Buffer %p with segment not in time format, "
         "not caching", buffer);
+    gst_buffer_unref (buffer);
     return;
   }