Avoid negation of signed integers in C that may hold INT*_MIN.

Reported by minoki.
Recent C compilers 'take advantage' of the undefined behavior.
This completely changes the meaning of expressions like (k == -k).

src/lj_ctype.c

@@ -577,7 +577,7 @@ GCstr *lj_ctype_repr_int64(lua_State *L, uint64_t n, int isunsigned)
   if (isunsigned) {
     *--p = 'U';
   } else if ((int64_t)n < 0) {
-    n = (uint64_t)-(int64_t)n;
+    n = ~n+1u;
     sign = 1;
   }
   do { *--p = (char)('0' + n % 10); } while (n /= 10);