wavparse: fix seeking in READY state
wavparse claims to be able to support seeking in the READY state by saving the pending seek event and actually seeking later after having parsed the header. Problem was that this seek event was reset on the READY -> PAUSED transition, making all this code useless. Fixing it by preserving the seek event during this transition.
Note that DTS marker detection isn't support in such scenario as gst_type_find_helper_for_buffer() needs a buffer containing the beginning of the stream.