play: Fix segfault if decoder is not supported.

play: Fix segfault if decoder is not supported.

When paraslash was build without libmad, para_play still recognizes mp3
files (because the mp3 audio format handler does not require libmad),
but decoding fails of course.

Specifically, the check_filter_arg() call in load_file() fails, but
the error handling code misses to clear the receiver node structure,
which results in a subsequent seqfault due to a double free.

This patch calls wipe_receiver_node() instead of open coding the
cleanup. This fixes the bug and actually simplifies the code a bit.