avcodec/wmaprodec: unbreak XMA mono decoding

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/wmaprodec.c

@@ -1755,6 +1755,7 @@ static int xma_decode_packet(AVCodecContext *avctx, void *data,
     if (got_stream_frame_ptr) {
         memcpy(&s->samples[s->current_stream * 2 + 0][s->offset[s->current_stream] * 512],
                s->frames[s->current_stream]->extended_data[0], 512 * 4);
+        if (avctx->channels > 1)
             memcpy(&s->samples[s->current_stream * 2 + 1][s->offset[s->current_stream] * 512],
                    s->frames[s->current_stream]->extended_data[1], 512 * 4);
         s->offset[s->current_stream]++;
@@ -1794,7 +1795,7 @@ static int xma_decode_packet(AVCodecContext *avctx, void *data,
             s->xma[i].skip_packets = FFMAX(0, s->xma[i].skip_packets - 1);
         }
 
-        for (i = 0; i < avctx->channels / 2; i++) {
+        for (i = 0; i < (avctx->channels + 1) / 2; i++) {
             offset = FFMIN(offset, s->offset[i]);
         }
 
@@ -1803,12 +1804,14 @@ static int xma_decode_packet(AVCodecContext *avctx, void *data,
             if ((bret = ff_get_buffer(avctx, frame, 0)) < 0)
                 return bret;
 
-            for (i = 0; i < avctx->channels / 2; i++) {
+            for (i = 0; i < (avctx->channels + 1) / 2; i++) {
                 memcpy(frame->extended_data[i * 2 + 0], s->samples[i * 2 + 0], frame->nb_samples * 4);
+                if (avctx->channels > 1)
                     memcpy(frame->extended_data[i * 2 + 1], s->samples[i * 2 + 1], frame->nb_samples * 4);
                 s->offset[i] -= offset;
                 if (s->offset[i]) {
                     memmove(s->samples[i * 2 + 0], s->samples[i * 2 + 0] + frame->nb_samples, s->offset[i] * 4 * 512);
+                    if (avctx->channels > 1)
                         memmove(s->samples[i * 2 + 1], s->samples[i * 2 + 1] + frame->nb_samples, s->offset[i] * 4 * 512);
                 }
             }