Array sizes in undump changed from unsigned to int

Array sizes are always int and are dumped as int, so there is no reason
to read them back as unsigned.

lmem.h

@@ -57,7 +57,8 @@
 #define luaM_freearray(L, b, n)   luaM_free_(L, (b), (n)*sizeof(*(b)))
 
 #define luaM_new(L,t)		cast(t*, luaM_malloc_(L, sizeof(t), 0))
-#define luaM_newvector(L,n,t)	cast(t*, luaM_malloc_(L, (n)*sizeof(t), 0))
+#define luaM_newvector(L,n,t)  \
+	cast(t*, luaM_malloc_(L, cast_sizet(n)*sizeof(t), 0))
 #define luaM_newvectorchecked(L,n,t) \
   (luaM_checksize(L,n,sizeof(t)), luaM_newvector(L,n,t))