Fixed a segfault in the DCA decoder with corrupted streams.

 It happens when the number of channels defined by DCAContext:acmod is lower
than DCAContext:prim_channels. In this case, dca_subsubframe() will call
qmf_32_subbands() using s->channel_order_tab[] entries equal to -1.

Originally committed as revision 22083 to svn://svn.ffmpeg.org/ffmpeg/trunk
