activate: move array allocation to heap

In theory 'n' could get quite large, and some sanitizers notice that,
let's hence avoid the stack, and use the heap instead.

Moreover, there's no need to include the first 3 fds in the array,
close_all() excludes those anyway.

See: #13064

src/activate/activate.c

@@ -49,8 +49,7 @@ static int add_epoll(int epoll_fd, int fd) {
 
 static int open_sockets(int *epoll_fd, bool accept) {
         char **address;
-        int n, fd, r;
-        int count = 0;
+        int n, fd, r, count = 0;
 
         n = sd_listen_fds(true);
         if (n < 0)
@@ -69,13 +68,18 @@ static int open_sockets(int *epoll_fd, bool accept) {
 
         /* Close logging and all other descriptors */
         if (arg_listen) {
-                int except[3 + n];
+                _cleanup_free_ int *except = NULL;
+                int i;
 
-                for (fd = 0; fd < SD_LISTEN_FDS_START + n; fd++)
-                        except[fd] = fd;
+                except = new(int, n);
+                if (!except)
+                        return log_oom();
+
+                for (i = 0; i < n; i++)
+                        except[i] = SD_LISTEN_FDS_START + i;
 
                 log_close();
-                r = close_all_fds(except, 3 + n);
+                r = close_all_fds(except, n);
                 if (r < 0)
                         return log_error_errno(r, "Failed to close all file descriptors: %m");
         }