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_carith.c

@@ -205,7 +205,7 @@ static int carith_int64(lua_State *L, CTState *cts, CDArith *ca, MMS mm)
       else
 	*up = lj_carith_powu64(u0, u1);
       break;
-    case MM_unm: *up = (uint64_t)-(int64_t)u0; break;
+    case MM_unm: *up = ~u0+1u; break;
     default: lua_assert(0); break;
     }
     lj_gc_check(L);