check_wav: Ask parent nodes before falling back to defaults.

check_wav: Ask parent nodes before falling back to defaults.

This instructs the ->execute handler of the check_wav task to ask its
parent nodes for the channel count, sample rate and sample format in
case no wav header was found and no value for the query was given at
the command line. Currently this can never succeed, since the only
parent node of the check_wav task is the stdin task, which has no
clue about these things. However, once check_wav is being used by
the resample filter, one parent might be a decoder which can tell.

This requires to add the new public btr_parent() to buffer_tree.c to
let the check_wav node obtain the node to start the search from.