core/exec-invoke: correct size for dont_close array in close_remaining_fds()

Follow-up for 79dd24cf14

src/core/exec-invoke.c

@@ -4293,7 +4293,7 @@ static int close_remaining_fds(
                 size_t n_fds) {
 
         size_t n_dont_close = 0;
-        int dont_close[n_fds + 17];
+        int dont_close[n_fds + 19];
 
         assert(params);
         assert(runtime);