wavpack: Check error codes rather than working around error conditions.

(cherry picked from commit dba2b63a98)

libavcodec/wavpack.c

@@ -1141,7 +1141,7 @@ static int wavpack_decode_block(AVCodecContext *avctx, int block_no,
             int16_t *dst = (int16_t*)samples + 1;
             int16_t *src = (int16_t*)samples;
             int cnt = samplecount;
-            while(cnt-- > 0){
+            while(cnt--){
                 *dst = *src;
                 src += channel_stride;
                 dst += channel_stride;
@@ -1150,7 +1150,7 @@ static int wavpack_decode_block(AVCodecContext *avctx, int block_no,
             int32_t *dst = (int32_t*)samples + 1;
             int32_t *src = (int32_t*)samples;
             int cnt = samplecount;
-            while(cnt-- > 0){
+            while(cnt--){
                 *dst = *src;
                 src += channel_stride;
                 dst += channel_stride;
@@ -1159,7 +1159,7 @@ static int wavpack_decode_block(AVCodecContext *avctx, int block_no,
             float *dst = (float*)samples + 1;
             float *src = (float*)samples;
             int cnt = samplecount;
-            while(cnt-- > 0){
+            while(cnt--){
                 *dst = *src;
                 src += channel_stride;
                 dst += channel_stride;