atdec: Fails to decode multichannel streams

Submitted by Andoni Alastruey @ylatuya

Link to original bug (#727757)

Description

Fails to decode http://mirror.bigbuckbunny.de/peach/bigbuckbunny_movies/big_buck_bunny_720p_h264.mov
Marked as a regression since it's the default decoder in OS X

iMac:Commands$ LC_ALL=C GST_DEBUG=atdec:5 ./gst-launch-1.0 playbin uri=file:///Users/fluendo/Downloads/big_buck_bunny_720p_h264.mov
Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
Redistribute latency...
0:00:00.189682000 54717 0x101079320 DEBUG atdec atdec.c:181:gst_atdec_start:<atdec0> start
0:00:00.189928000 54717 0x101079320 DEBUG atdec atdec.c:271:gst_atdec_set_format:<atdec0> set_format
0:00:00.193547000 54717 0x101079320 ERROR default audio-channels.c:355:gst_audio_channel_positions_from_mask: Invalid channel mask 0x0000000000000003 for 6 channels
0:00:00.193624000 54717 0x101079320 ERROR default audio-info.c:286:gst_audio_info_from_caps: Invalid channel mask 0x0000000000000003 for 6 channels
2014-04-07 16:36:49.133 gst-launch-1.0[54717:1403] 16:36:49.133 ERROR: [0x102195000] >aqme> MESubmixGraph.h:218: SetMixerChannelLayout: (0x8d5bd060): scope 2, element 0, tag=0x650002: error -10851
0:00:00.199654000 54717 0x101079320 WARN atdec atdec.c:331:gst_atdec_set_format:<atdec0> error: AudioQueueSetOfflineRenderFormat returned error: 560226676
ERROR: from element /GstPlayBin:playbin0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0/GstATDec:atdec0: The stream is in the wrong format.
Additional debug info:
atdec.c(331): gst_atdec_set_format (): /GstPlayBin:playbin0/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0/GstATDec:atdec0:
AudioQueueSetOfflineRenderFormat returned error: 560226676
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...