gst/matroska/ebml-write.*: Make sure we send a newsegment event in BYTES format before sending buffers (#328531).

Original commit message from CVS:
* gst/matroska/ebml-write.c: (gst_ebml_write_reset),
(gst_ebml_write_flush_cache), (gst_ebml_write_element_push),
(gst_ebml_write_seek):
* gst/matroska/ebml-write.h:
Make sure we send a newsegment event in BYTES format
before sending buffers (#328531).