Avoid underflows in 'luaF_getlocalname'

A call like 'debug.getlocal(test, -2147483648)' could decrement
-2147483648, causing an underflow. (As this happens in a debug function,
the fix was not strictly necessary, but it has no cost in the generated
code.)

ltests.c

@@ -853,7 +853,7 @@ static int listlocals (lua_State *L) {
   luaL_argcheck(L, lua_isfunction(L, 1) && !lua_iscfunction(L, 1),
                  1, "Lua function expected");
   p = getproto(obj_at(L, 1));
-  while ((name = luaF_getlocalname(p, ++i, pc)) != NULL)
+  while ((name = luaF_getlocalname(p, cast_uint(++i), pc)) != NULL)
     lua_pushstring(L, name);
   return i-1;
 }