Clear transition mode array for all subbands.

Number of XBR subbands might be theoretically greater than core. With
XBR transition mode enabled stale core transition mode might be
referenced. Fix XBR subframe parsing in this case by zeroing entire
transition mode array beforehand.

libdcadec/core_decoder.c

@@ -410,7 +410,7 @@ static int parse_subframe_header(struct core_decoder *core, int sf,
     // Transition mode
     for (ch = xch_base; ch < core->nchannels; ch++) {
         // Clear transition mode for all subbands
-        for (band = 0; band < core->nsubbands[ch]; band++)
+        for (band = 0; band < MAX_SUBBANDS; band++)
             core->transition_mode[sf][ch][band] = 0;
 
         // Transient possible only if more than one subsubframe