sysext: fix a memory leak

Introduced by 41712cd.

=================================================================
==2194==ERROR: LeakSanitizer: detected memory leaks
Indirect leak of 359856 byte(s) in 459 object(s) allocated from:
    #0 0x7ffff7511df4  (/usr/lib64/clang/16/lib/linux/libclang_rt.asan-powerpc64le.so+0x191df4) (BuildId: 47e1dd371a2b8525b6cb737760a4dc535f30ea10)
    #1 0x7ffff6bb5fb0 in message_from_header /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/bus-message.c:372:13
    #2 0x7ffff6bb5fb0 in bus_message_from_malloc /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/bus-message.c:421:13
    #3 0x7ffff6c23f54 in bus_socket_make_message /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/bus-socket.c:1222:13
    systemd#4 0x7ffff6c22d10 in bus_socket_read_message /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/bus-socket.c
    systemd#5 0x7ffff6c4d414 in bus_read_message /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/sd-bus.c:2082:16
    systemd#6 0x7ffff6c4d414 in sd_bus_call /systemd-meson-build/../root/systemd/src/libsystemd/sd-bus/sd-bus.c:2480:21
    systemd#7 0x7ffff6682904 in bus_service_manager_reload /systemd-meson-build/../root/systemd/src/shared/bus-unit-util.c:2823:13
    systemd#8 0x1000d570 in daemon_reload /systemd-meson-build/../root/systemd/src/sysext/sysext.c:233:16
    systemd#9 0x100090f8 in merge /systemd-meson-build/../root/systemd/src/sysext/sysext.c:895:21
    systemd#10 0x10006ff4 in verb_merge /systemd-meson-build/../root/systemd/src/sysext/sysext.c:964:16
    systemd#11 0x7ffff69ae894 in dispatch_verb /systemd-meson-build/../root/systemd/src/shared/verbs.c:103:24
    systemd#12 0x10004570 in sysext_main /systemd-meson-build/../root/systemd/src/sysext/sysext.c:1194:16
    systemd#13 0x10004570 in run /systemd-meson-build/../root/systemd/src/sysext/sysext.c:1214:16
    systemd#14 0x10004570 in main /systemd-meson-build/../root/systemd/src/sysext/sysext.c:1217:1
    systemd#15 0x7ffff5f5a968 in generic_start_main.isra.0 (/lib64/libc.so.6+0x2a968) (BuildId: c218e04818632a05c23f6fdcca16f93e95ea7de2)
    systemd#16 0x7ffff5f5ab00 in __libc_start_main (/lib64/libc.so.6+0x2ab00) (BuildId: c218e04818632a05c23f6fdcca16f93e95ea7de2)
Indirect leak of 124984 byte(s) in 459 object(s) allocated from:
...
    systemd#11 0x7ffff5f5a968 in generic_start_main.isra.0 (/lib64/libc.so.6+0x2a968) (BuildId: c218e04818632a05c23f6fdcca16f93e95ea7de2)
    systemd#12 0x7ffff5f5ab00 in __libc_start_main (/lib64/libc.so.6+0x2ab00) (BuildId: c218e04818632a05c23f6fdcca16f93e95ea7de2)
SUMMARY: AddressSanitizer: 493766 byte(s) leaked in 1383 allocation(s).

src/sysext/sysext.c

@@ -223,7 +223,7 @@ static int need_reload(void) {
 }
 
 static int daemon_reload(void) {
-        sd_bus *bus;
+         _cleanup_(sd_bus_flush_close_unrefp) sd_bus *bus = NULL;
         int r;
 
         r = bus_connect_system_systemd(&bus);