fix Av1FrameHeader.tiling.n_bytes sign compare warning

src/decode.c

@@ -2524,7 +2524,7 @@ int dav1d_decode_frame(Dav1dFrameContext *const f) {
             } else {
                 if (f->frame_hdr.tiling.n_bytes > size) goto error;
                 tile_sz = 0;
-                for (int k = 0; k < f->frame_hdr.tiling.n_bytes; k++)
+                for (unsigned k = 0; k < f->frame_hdr.tiling.n_bytes; k++)
                     tile_sz |= *data++ << (k * 8);
                 tile_sz++;
                 size -= f->frame_hdr.tiling.n_bytes;

src/levels.h

@@ -440,7 +440,7 @@ typedef struct Av1FrameHeader {
     int refresh_context;
     struct {
         int uniform;
-        int n_bytes;
+        unsigned n_bytes;
         int min_log2_cols, max_log2_cols, log2_cols, cols;
         int col_start_sb[1025];
         int min_log2_rows, max_log2_rows, log2_rows, rows;