busctl: avoid asserting on NULL message

Avoid passing a NULL message to sd_bus_message_is_signal(), to not trip
over an assertion:

[  132.869436] H testsuite-82.sh[614]: + systemctl --no-block --check-inhibitors=yes soft-reboot
[  132.967386] H systemd[1]: Created slice system-systemd\x2dcoredump.slice.
[  133.018292] H systemd[1]: Starting inhibit.service...
[  133.122610] H systemd[1]: Started systemd-coredump@0-665-0.service.
[  133.163643] H systemd[1]: Started inhibit.service.
[  133.206836] H testsuite-82.sh[614]: + exec sleep infinity
[  133.236762] H systemd-logind[611]: The system will reboot now!
[  135.891607] H systemd-coredump[667]: [🡕] Process 663 (busctl) of user 0 dumped core.

                                        Stack trace of thread 663:
                                        #0  0x00007f2ec45e6acf raise (libc.so.6 + 0x4eacf)
                                        #1  0x00007f2ec45b9ea5 abort (libc.so.6 + 0x21ea5)
                                        #2  0x00007f2ec4b5c9a6 log_assert_failed (libsystemd-shared-255.so + 0x1ff9a6)
                                        #3  0x00007f2ec4b5dca5 log_assert_failed_return (libsystemd-shared-255.so + 0x200ca5)
                                        #4  0x00007f2ec4bb3df6 sd_bus_message_is_signal (libsystemd-shared-255.so + 0x256df6)
                                        #5  0x000000000040e478 monitor (busctl + 0xe478)
                                        #6  0x000000000040e82f verb_monitor (busctl + 0xe82f)
                                        #7  0x00007f2ec4b202cb dispatch_verb (libsystemd-shared-255.so + 0x1c32cb)
                                        #8  0x00000000004074fa busctl_main (busctl + 0x74fa)
                                        #9  0x0000000000407525 run (busctl + 0x7525)
                                        #10 0x000000000040ff67 main (busctl + 0xff67)
                                        #11 0x00007f2ec45d2d85 __libc_start_main (libc.so.6 + 0x3ad85)
                                        #12 0x00000000004044be _start (busctl + 0x44be)
                                        ELF object binary architecture: AMD x86-64
[  136.141152] H dbus-daemon[634]: [system] Monitoring connection :1.2 closed.
[  136.152233] H systemd[1]: busctl.service: Main process exited, code=dumped, status=6/ABRT
[  136.153996] H systemd[1]: busctl.service: Failed with result 'core-dump'.

The asertion in question:

Assertion 'm' failed at src/libsystemd/sd-bus/bus-message.c:1015, function sd_bus_message_is_signal(). Aborting.

We can get a NULL message here through sd_bus_process() ->
bus_process_internal() -> process_running(), so let's handle this case
appropriately.

src/busctl/busctl.c

@@ -1320,24 +1320,24 @@ static int monitor(int argc, char **argv, int (*dump)(sd_bus_message *m, FILE *f
                 if (r < 0)
                         return log_error_errno(r, "Failed to process bus: %m");
 
-                if (!is_monitor) {
-                        const char *name;
+                if (m) {
+                        if (!is_monitor) {
+                                const char *name;
 
-                        /* wait until we lose our unique name */
-                        if (sd_bus_message_is_signal(m, "org.freedesktop.DBus", "NameLost") <= 0)
-                                continue;
+                                /* wait until we lose our unique name */
+                                if (sd_bus_message_is_signal(m, "org.freedesktop.DBus", "NameLost") <= 0)
+                                        continue;
 
-                        r = sd_bus_message_read(m, "s", &name);
-                        if (r < 0)
-                                return bus_log_parse_error(r);
+                                r = sd_bus_message_read(m, "s", &name);
+                                if (r < 0)
+                                        return bus_log_parse_error(r);
 
-                        if (streq(name, unique_name))
-                                is_monitor = true;
+                                if (streq(name, unique_name))
+                                        is_monitor = true;
 
-                        continue;
-                }
+                                continue;
+                        }
 
-                if (m) {
                         dump(m, stdout);
                         fflush(stdout);