nspawn: call json_dispatch() with a correct pointer

nspawn: call json_dispatch() with a correct pointer

Otherwise hilarity ensues:

 AddressSanitizer:DEADLYSIGNAL
 =================================================================
 ==722==ERROR: AddressSanitizer: SEGV on unknown address 0xffffffff00000000 (pc 0x7f8d50ca9ffb bp 0x7fff11b0d4a0 sp 0x7fff11b0cc30 T0)
 ==722==The signal is caused by a READ memory access.
     #0 0x7f8d50ca9ffb in __interceptor_strcmp.part.0 (/lib64/libasan.so.8+0xa9ffb)
     #1 0x7f8d4f9cf5a1 in strcmp_ptr ../src/fundamental/string-util-fundamental.h:33
     #2 0x7f8d4f9cf5f8 in streq_ptr ../src/fundamental/string-util-fundamental.h:46
     #3 0x7f8d4f9d74d2 in free_and_strdup ../src/basic/string-util.c:948
     #4 0x49139a in free_and_strdup_warn ../src/basic/string-util.h:197
     #5 0x4923eb in oci_absolute_path ../src/nspawn/nspawn-oci.c:139
     #6 0x7f8d4f6bd359 in json_dispatch ../src/shared/json.c:4395
     #7 0x4a8831 in oci_hooks_array ../src/nspawn/nspawn-oci.c:2089
     #8 0x7f8d4f6bd359 in json_dispatch ../src/shared/json.c:4395
     #9 0x4a8b56 in oci_hooks ../src/nspawn/nspawn-oci.c:2112
     #10 0x7f8d4f6bd359 in json_dispatch ../src/shared/json.c:4395
     #11 0x4aa298 in oci_load ../src/nspawn/nspawn-oci.c:2197
     #12 0x446cec in load_oci_bundle ../src/nspawn/nspawn.c:4744
     #13 0x44ffa7 in run ../src/nspawn/nspawn.c:5477
     #14 0x4552fb in main ../src/nspawn/nspawn.c:5920
     #15 0x7f8d4e04a50f in __libc_start_call_main (/lib64/libc.so.6+0x2750f)
     #16 0x7f8d4e04a5c8 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x275c8)
     #17 0x40d284 in _start (/usr/bin/systemd-nspawn+0x40d284)
 AddressSanitizer can not provide additional info.
 SUMMARY: AddressSanitizer: SEGV (/lib64/libasan.so.8+0xa9ffb) in __interceptor_strcmp.part.0
 ==722==ABORTING