pcm: fix decoding of pcm_s16le_planar on big-endian

The sample count is decremented by the DECODE() macro and needs to be reset
in each loop iteration. Also, DECODE() increments the src pointer so that does
not need to be done separately.

libavcodec/pcm.c

@@ -338,15 +338,17 @@ static int pcm_decode_frame(AVCodecContext *avctx, void *data,
         break;
     case AV_CODEC_ID_PCM_S16LE_PLANAR:
     {
+        int av_unused n2;
         n /= avctx->channels;
         for (c = 0; c < avctx->channels; c++) {
             samples = s->frame.extended_data[c];
 #if HAVE_BIGENDIAN
-            DECODE(16, le16, src, samples, n, 0, 0)
+            n2 = n;
+            DECODE(16, le16, src, samples, n2, 0, 0)
 #else
             memcpy(samples, src, n * 2);
-#endif
             src += n * 2;
+#endif
         }
         break;
     }