resample: Simplify initialization().

In resample_init(), the first check removed in this commit was bogus
because in case there is no parent buffer tree node, we must only
abort if there is no input pending either.

In resample_post_select(), we move up the check of the node status
so that we now call resample_init() only after we know that there
is input available. This makes the second check in resample_init()
pointless as the condition can never be true.