random.c: sipseed_keys_t

* random.c (sipseed): separate type of keys to reduce use of the
  magic number.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54928 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

random.c

@@ -1455,9 +1455,10 @@ random_s_rand(int argc, VALUE *argv, VALUE obj)
 #include "siphash.c"
 
 static st_index_t hashseed;
+typedef uint8_t sipseed_keys_t[16];
 static union {
-    uint8_t key[16];
-    uint32_t u32[(16 * sizeof(uint8_t) - 1) / sizeof(uint32_t)];
+    sipseed_keys_t key;
+    uint32_t u32[type_roomof(sipseed_keys_t, uint32_t)];
 } sipseed;
 
 static void