nspawn: fix a global-buffer-overflow

Whoopsie.

=================================================================
==3789231==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000051d0b8 at pc 0x7f70850bc904 bp 0x7ffd9bbdf660 sp 0x7ffd9bbdf658
READ of size 8 at 0x00000051d0b8 thread T0
    #0 0x7f70850bc903 in json_dispatch ../src/shared/json.c:4347
    systemd#1 0x4a5b54 in oci_seccomp_syscalls ../src/nspawn/nspawn-oci.c:1838
    systemd#2 0x7f70850bd359 in json_dispatch ../src/shared/json.c:4395
    systemd#3 0x4a668c in oci_seccomp ../src/nspawn/nspawn-oci.c:1905
    systemd#4 0x7f70850bd359 in json_dispatch ../src/shared/json.c:4395
    systemd#5 0x4a7d8c in oci_linux ../src/nspawn/nspawn-oci.c:2030
    systemd#6 0x7f70850bd359 in json_dispatch ../src/shared/json.c:4395
    systemd#7 0x4aa31c in oci_load ../src/nspawn/nspawn-oci.c:2198
    systemd#8 0x446cec in load_oci_bundle ../src/nspawn/nspawn.c:4744
    systemd#9 0x44ffa7 in run ../src/nspawn/nspawn.c:5477
    systemd#10 0x4552fb in main ../src/nspawn/nspawn.c:5920
    systemd#11 0x7f7083a4a50f in __libc_start_call_main (/lib64/libc.so.6+0x2750f)
    systemd#12 0x7f7083a4a5c8 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x275c8)
    systemd#13 0x40d284 in _start (/home/fsumsal/repos/@systemd/systemd/build-san/systemd-nspawn+0x40d284)

0x00000051d0b8 is located 40 bytes to the left of global variable 'bus_standard_errors_copy_0' defined in '../src/libsystemd/sd-bus/bus-error.h:57:1' (0x51d0e0) of size 8
0x00000051d0b8 is located 0 bytes to the right of global variable 'table' defined in '../src/nspawn/nspawn-oci.c:1829:43' (0x51d040) of size 120
SUMMARY: AddressSanitizer: global-buffer-overflow ../src/shared/json.c:4347 in json_dispatch
Shadow bytes around the buggy address:
  0x00008009b9c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009b9d0: 00 00 00 00 f9 f9 f9 f9 00 00 00 00 00 00 00 00
  0x00008009b9e0: 00 00 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
  0x00008009b9f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009ba00: 00 f9 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
=>0x00008009ba10: 00 00 00 00 00 00 00[f9]f9 f9 f9 f9 00 f9 f9 f9
  0x00008009ba20: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009ba30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009ba40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009ba50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008009ba60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==3789231==ABORTING

src/nspawn/nspawn-oci.c

@@ -1829,6 +1829,7 @@ static int oci_seccomp_syscalls(const char *name, JsonVariant *v, JsonDispatchFl
                         { "names",  JSON_VARIANT_ARRAY,  json_dispatch_strv, offsetof(struct syscall_rule, names),  JSON_MANDATORY },
                         { "action", JSON_VARIANT_STRING, oci_seccomp_action, offsetof(struct syscall_rule, action), JSON_MANDATORY },
                         { "args",   JSON_VARIANT_ARRAY,  oci_seccomp_args,   0,                                     0              },
+                        {}
                 };
                 struct syscall_rule rule = {
                         .action = UINT32_MAX,