resolve: avoid memory leak from a partially processed RR

==5==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 4096 byte(s) in 1 object(s) allocated from:
    #0 0x4a2056 in __interceptor_malloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:69:3
    #1 0x5180a9 in malloc (/build/fuzz-resource-record+0x5180a9)
    #2 0x4f7182 in dns_packet_extend /work/build/../../src/systemd/src/resolve/resolved-dns-packet.c:371:36
    #3 0x4f8b8b in dns_packet_append_uint8 /work/build/../../src/systemd/src/resolve/resolved-dns-packet.c:433:13
    #4 0x4f8b8b in dns_packet_append_name /work/build/../../src/systemd/src/resolve/resolved-dns-packet.c:597:13
    #5 0x4f8f16 in dns_packet_append_key /work/build/../../src/systemd/src/resolve/resolved-dns-packet.c:622:13
    #6 0x4fa9a0 in dns_packet_append_rr /work/build/../../src/systemd/src/resolve/resolved-dns-packet.c:883:13
    #7 0x4eb00c in dns_resource_record_to_wire_format /work/build/../../src/systemd/src/resolve/resolved-dns-rr.c:1224:13
    #8 0x4df7be in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/resolve/fuzz-resource-record.c:32:16
    #9 0x518428 in NaloFuzzerTestOneInput (/build/fuzz-resource-record+0x518428)
    #10 0x537433 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #11 0x536c1a in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:514:3
    #12 0x5382e9 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    #13 0x538fb5 in fuzzer::Fuzzer::Loop(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:895:5
    #14 0x52831f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #15 0x528be8 in LLVMFuzzerRunDriver /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:925:10
    #16 0x5186a5 in main (/build/fuzz-resource-record+0x5186a5)
    #17 0x7f991fab8082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)

DEDUP_TOKEN: __interceptor_malloc--malloc--dns_packet_extend
SUMMARY: AddressSanitizer: 4096 byte(s) leaked in 1 allocation(s).

Found by Nallocfuzz.

(cherry picked from commit b453ebf)
(cherry picked from commit 9dde31a)

src/resolve/resolved-dns-rr.c

@@ -1222,8 +1222,10 @@ int dns_resource_record_to_wire_format(DnsResourceRecord *rr, bool canonical) {
                 return 0;
 
         r = dns_packet_append_rr(&packet, rr, 0, &start, &rds);
-        if (r < 0)
+        if (r < 0) {
+                dns_packet_unref(&packet);
                 return r;
+        }
 
         assert(start == 0);
         assert(packet._data);