make narrowing conversion explicit

PiperOrigin-RevId: 914775613

c/dec/decode.c

diff --git a/c/dec/decode.c b/c/dec/decode.c
index 4a8fcfe..b579e33 100644
--- a/c/dec/decode.c
+++ b/c/dec/decode.c
@@ -1554,7 +1554,7 @@
   }
   addon->chunks[addon->num_chunks] = data;
   addon->num_chunks++;
-  addon->total_size += size;
+  addon->total_size += (uint32_t)size;
   addon->chunk_offsets[addon->num_chunks] = addon->total_size;
   return BROTLI_TRUE;
 }