fuzz-lldp-rx: fuzz lldp_rx_build_neighbors_json()

Addresses https://github.com/systemd/systemd/pull/31583#discussion_r1509882453.

src/libsystemd-network/fuzz-lldp-rx.c

@@ -9,6 +9,7 @@
 #include "fd-util.h"
 #include "fuzz.h"
 #include "lldp-network.h"
+#include "lldp-rx-internal.h"
 
 static int test_fd[2] = EBADF_PAIR;
 
@@ -22,6 +23,7 @@ int lldp_network_bind_raw_socket(int ifindex) {
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_(sd_event_unrefp) sd_event *e = NULL;
         _cleanup_(sd_lldp_rx_unrefp) sd_lldp_rx *lldp_rx = NULL;
+        _cleanup_(json_variant_unrefp) JsonVariant *v = NULL;
 
         if (outside_size_range(size, 0, 2048))
                 return 0;
@@ -37,6 +39,8 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         assert_se(write(test_fd[1], data, size) == (ssize_t) size);
         assert_se(sd_event_run(e, 0) >= 0);
 
+        assert_se(lldp_rx_build_neighbors_json(lldp_rx, &v) >= 0);
+
         assert_se(sd_lldp_rx_stop(lldp_rx) >= 0);
         assert_se(sd_lldp_rx_detach_event(lldp_rx) >= 0);
         test_fd[1] = safe_close(test_fd[1]);