busctl: avoid asserting on NULL message

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.