tests: add a file triggering "applying zero offset to null pointer"

```
../src/basic/hexdecoct.c:66:44: runtime error: applying zero offset to null pointer
    #0 0x7f6022650c44 in hexmem /home/vagrant/systemd/build-fuzzers/../src/basic/hexdecoct.c:66:44
    #1 0x577583 in dns_resource_record_to_string /home/vagrant/systemd/build-fuzzers/../src/resolve/resolved-dns-rr.c:1140:21
    #2 0x563669 in LLVMFuzzerTestOneInput /home/vagrant/systemd/build-fuzzers/../src/resolve/fuzz-resource-record.c:25:39
    #3 0x44d2a1 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/vagrant/systemd/out/fuzz-resource-record+0x44d2a1) (BuildId: 88135c111396e9441a475302ccabd2f9a58c7e89)
    #4 0x42d32f in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) (/home/vagrant/systemd/out/fuzz-resource-record+0x42d32f) (BuildId: 88135c111396e9441a475302ccabd2f9a58c7e89)
    #5 0x434920 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/vagrant/systemd/out/fuzz-resource-record+0x434920) (BuildId: 88135c111396e9441a475302ccabd2f9a58c7e89)
    #6 0x424006 in main (/home/vagrant/systemd/out/fuzz-resource-record+0x424006) (BuildId: 88135c111396e9441a475302ccabd2f9a58c7e89)
    #7 0x7f602142950f in __libc_start_call_main (/lib64/libc.so.6+0x2950f) (BuildId: 85c438f4ff93e21675ff174371c9c583dca00b2c)
    #8 0x7f60214295c8 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x295c8) (BuildId: 85c438f4ff93e21675ff174371c9c583dca00b2c)
    #9 0x424044 in _start (/home/vagrant/systemd/out/fuzz-resource-record+0x424044) (BuildId: 88135c111396e9441a475302ccabd2f9a58c7e89)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/basic/hexdecoct.c:66:44 in
```

diff --git a/test/fuzz/fuzz-resource-record/ub-zero-length-rdata b/test/fuzz/fuzz-resource-record/ub-zero-length-rdata
new file mode 100644 (file)
index 0000000..b5301ee
Binary files /dev/null and b/test/fuzz/fuzz-resource-record/ub-zero-length-rdata differ