output/pulse: use pa_channel_map_init_extend() instead of _auto()

Unlike pa_channel_map_init_auto(), pa_channel_map_init_extend() does
not fail if there is no valid mapping for the given channel count, but
instead maps additional "AUX" channels.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/493

NEWS

@@ -7,6 +7,8 @@ ver 0.21.6 (not yet released)
   - cdio_paranoia: fix build failure due to missing #include
 * decoder
   - opus: fix replay gain when there are no other tags
+* output
+  - pulse: work around error with unusual channel count
 * playlist
   - flac: fix use-after-free bug
 * support abstract sockets on Linux

src/output/plugins/PulseOutputPlugin.cxx

@@ -581,8 +581,8 @@ PulseOutput::SetupStream(const pa_sample_spec &ss)
 
 	/* WAVE-EX is been adopted as the speaker map for most media files */
 	pa_channel_map chan_map;
-	pa_channel_map_init_auto(&chan_map, ss.channels,
-				 PA_CHANNEL_MAP_WAVEEX);
+	pa_channel_map_init_extend(&chan_map, ss.channels,
+				   PA_CHANNEL_MAP_WAVEEX);
 	stream = pa_stream_new(context, name, &ss, &chan_map);
 	if (stream == nullptr)
 		throw MakePulseError(context,