sd-network: avoid leaking DHCPLease

sd-network: avoid leaking DHCPLease

If we fail any allocation prior adding the lease to the server lease
hashmap.

==2103==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 128 byte(s) in 2 object(s) allocated from:
    #0 0x4a203e in __interceptor_calloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:77:3
    #1 0x4f6341 in calloc (/build/fuzz-dhcp-server+0x4f6341)
    #2 0x4ec818 in add_lease /work/build/../../src/systemd/src/libsystemd-network/fuzz-dhcp-server.c:26:9
    #3 0x4ec2bf in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/libsystemd-network/fuzz-dhcp-server.c:75:9
    #4 0x4f68a8 in NaloFuzzerTestOneInput (/build/fuzz-dhcp-server+0x4f68a8)
    #5 0x5158b3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #6 0x51509a 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
    #7 0x516769 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    #8 0x517435 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
    #9 0x50679f in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #10 0x507068 in LLVMFuzzerRunDriver /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:925:10
    #11 0x4f6b25 in main (/build/fuzz-dhcp-server+0x4f6b25)
    #12 0x7f16084e3082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)

DEDUP_TOKEN: __interceptor_calloc--calloc--add_lease
SUMMARY: AddressSanitizer: 128 byte(s) leaked in 2 allocation(s).

Found by Nallocufzz.