aacdec: fix "may be used uninitialized" warning

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/aacdec.c

@@ -572,6 +572,8 @@ static void decode_channel_map(uint8_t layout_map[][3],
         case AAC_CHANNEL_LFE:
             syn_ele = TYPE_LFE;
             break;
+        default:
+            av_assert0(0);
         }
         layout_map[0][0] = syn_ele;
         layout_map[0][1] = get_bits(gb, 4);