ext/wavpack/: Improve discont handling by checking if the next Wavpack block has the expected, following block index.

Original commit message from CVS:
* ext/wavpack/gstwavpackdec.c: (gst_wavpack_dec_reset),
(gst_wavpack_dec_chain):
* ext/wavpack/gstwavpackdec.h:
* ext/wavpack/gstwavpackparse.c: (gst_wavpack_parse_reset),
(gst_wavpack_parse_push_buffer):
* ext/wavpack/gstwavpackparse.h:
Improve discont handling by checking if the next Wavpack block has
the expected, following block index.