sd-device: check fd validity before using in sendmsg in test

CID#1663676

Follow-up for 7003a9bf09

src/libsystemd/sd-device/test-sd-device-monitor.c

@@ -8,6 +8,7 @@
 #include "device-monitor-private.h"
 #include "device-private.h"
 #include "device-util.h"
+#include "fd-util.h"
 #include "io-util.h"
 #include "iovec-util.h"
 #include "mountpoint-util.h"
@@ -444,7 +445,9 @@ TEST(sd_device_monitor_receive_bad_properties_off) {
                 .msg_name = &sa,
                 .msg_namelen = sizeof(struct sockaddr_nl),
         };
-        ASSERT_OK_ERRNO(sendmsg(sd_device_monitor_get_fd(monitor_server), &smsg, 0));
+
+        int fd = ASSERT_FD(sd_device_monitor_get_fd(monitor_server));
+        ASSERT_OK_ERRNO(sendmsg(fd, &smsg, 0));
 
         _cleanup_(sd_device_unrefp) sd_device *dev = NULL;
         ASSERT_ERROR(sd_device_monitor_receive(monitor_client, &dev), EAGAIN);