shorten: use the unsigned type where needed

get_uint returns an unsigned value, use an unsigned to store
blocksize to make sure the comparison logic is correct and report
correctly the error for the channel count not supported.

libavcodec/shorten.c

@@ -83,7 +83,7 @@ typedef struct ShortenContext {
     GetBitContext gb;
 
     int min_framesize, max_framesize;
-    int channels;
+    unsigned channels;
 
     int32_t *decoded[MAX_CHANNELS];
     int32_t *decoded_base[MAX_CHANNELS];
@@ -339,7 +339,11 @@ static int read_header(ShortenContext *s)
     s->internal_ftype = get_uint(s, TYPESIZE);
 
     s->channels = get_uint(s, CHANSIZE);
-    if (s->channels <= 0 || s->channels > MAX_CHANNELS) {
+    if (!s->channels) {
+        av_log(s->avctx, AV_LOG_ERROR, "No channels reported\n");
+        return AVERROR_INVALIDDATA;
+    }
+    if (s->channels > MAX_CHANNELS) {
         av_log(s->avctx, AV_LOG_ERROR, "too many channels: %d\n", s->channels);
         s->channels = 0;
         return AVERROR_INVALIDDATA;
@@ -348,7 +352,8 @@ static int read_header(ShortenContext *s)
 
     /* get blocksize if version > 0 */
     if (s->version > 0) {
-        int skip_bytes, blocksize;
+        int skip_bytes;
+        unsigned blocksize;
 
         blocksize = get_uint(s, av_log2(DEFAULT_BLOCK_SIZE));
         if (!blocksize || blocksize > MAX_BLOCKSIZE) {
@@ -501,7 +506,7 @@ static int shorten_decode_frame(AVCodecContext *avctx, void *data,
                 s->bitshift = get_ur_golomb_shorten(&s->gb, BITSHIFTSIZE);
                 break;
             case FN_BLOCKSIZE: {
-                int blocksize = get_uint(s, av_log2(s->blocksize));
+                unsigned blocksize = get_uint(s, av_log2(s->blocksize));
                 if (blocksize > s->blocksize) {
                     av_log(avctx, AV_LOG_ERROR,
                            "Increasing block size is not supported\n");