nspawn: avoid NULL pointer dereference

nspawn: avoid NULL pointer dereference

When merging the settings we take the pointer to the array of extra
devices, but don't reset the array counter to zero. This later leads to
a NULL pointer dereference, where device_node_array_free() attempts to
loop over a NULL pointer:

+ systemd-nspawn --oci-bundle=/var/lib/machines/testsuite-13.oci-bundle.Npo
 ../src/nspawn/nspawn-settings.c:118:29: runtime error: member access within null pointer of type 'struct DeviceNode'
     #0 0x4b91ee in device_node_array_free ../src/nspawn/nspawn-settings.c:118
     #1 0x4ba42a in settings_free ../src/nspawn/nspawn-settings.c:161
     #2 0x410b79 in settings_freep ../src/nspawn/nspawn-settings.h:249
     #3 0x446ce8 in load_oci_bundle ../src/nspawn/nspawn.c:4733
     #4 0x44ff42 in run ../src/nspawn/nspawn.c:5476
     #5 0x455296 in main ../src/nspawn/nspawn.c:5919
     #6 0x7f0cb7a4a50f in __libc_start_call_main (/lib64/libc.so.6+0x2750f)
     #7 0x7f0cb7a4a5c8 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x275c8)
     #8 0x40d284 in _start (/usr/bin/systemd-nspawn+0x40d284)
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/nspawn/nspawn-settings.c:118:29 in

Also, add an appropriate assert to catch such issues in the future.