10l: do not try to unpack DTX frames in AMR-NB decoder

Originally committed as revision 22930 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/amrnbdec.c

@@ -195,7 +195,7 @@ static enum Mode unpack_bitstream(AMRContext *p, const uint8_t *buf,
     p->bad_frame_indicator = !get_bits1(&gb); // quality bit
     skip_bits(&gb, 2);                        // two padding bits
 
-    if (mode <= MODE_DTX) {
+    if (mode < MODE_DTX) {
         uint16_t *data = (uint16_t *)&p->frame;
         const uint8_t *order = amr_unpacking_bitmaps_per_mode[mode];
         int field_size;