jdhuff.h: Fix ASan regression caused by 8fa70367

The 0xFF is, in fact, necessary.
diff --git a/jdhuff.h b/jdhuff.h
index 60d8e8b..cfa0b7f 100644
--- a/jdhuff.h
+++ b/jdhuff.h
@@ -237,7 +237,7 @@
     if (nb > 16) \
       s = 0; \
     else \
-      s = htbl->pub->huffval[(int)(s + htbl->valoffset[nb])]; \
+      s = htbl->pub->huffval[(int)(s + htbl->valoffset[nb]) & 0xFF]; \
   }
 
 /* Out-of-line case for Huffman code fetching */