bus-message: avoid dereferencing a NULL pointer

We'd try to map a zero-byte buffer from a NULL pointer, which is undefined behaviour.

src/systemd/src/libsystemd/sd-bus/bus-message.c:3161:60: runtime error: applying zero offset to null pointer
    #0 0x7f6ff064e691 in find_part /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-message.c:3161:60
    #1 0x7f6ff0640788 in message_peek_body /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-message.c:3283:16
    #2 0x7f6ff064e8db in enter_struct_or_dict_entry /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-message.c:3967:21
    #3 0x7f6ff06444ac in bus_message_enter_struct /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-message.c:4009:13
    #4 0x7f6ff0641dde in sd_bus_message_enter_container /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-message.c:4136:21
    #5 0x7f6ff0619874 in sd_bus_message_dump /work/build/../../src/systemd/src/libsystemd/sd-bus/bus-dump.c:178:29
    #6 0x4293d9 in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-bus-message.c:39:9
    #7 0x441986 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/libfuzzer/FuzzerLoop.cpp:558:15
    #8 0x44121e in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool*) /src/libfuzzer/FuzzerLoop.cpp:470:3
    #9 0x443164 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__1::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) /src/libfuzzer/FuzzerLoop.cpp:770:7
    #10 0x4434bc in fuzzer::Fuzzer::Loop(std::__1::vector<fuzzer::SizedFile, fuzzer::fuzzer_allocator<fuzzer::SizedFile> >&) /src/libfuzzer/FuzzerLoop.cpp:799:3
    #11 0x42d2bc in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/libfuzzer/FuzzerDriver.cpp:846:6
    #12 0x42978a in main /src/libfuzzer/FuzzerMain.cpp:19:10
    #13 0x7f6fef13c82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #14 0x407808 in _start (out/fuzz-bus-message+0x407808)

src/libsystemd/sd-bus/bus-message.c

@@ -3159,7 +3159,8 @@ static struct bus_body_part* find_part(sd_bus_message *m, size_t index, size_t s
                                 return NULL;
 
                         if (p)
-                                *p = (uint8_t*) part->data + index - begin;
+                                *p = part->data ? (uint8_t*) part->data + index - begin
+                                        : NULL; /* Avoid dereferencing a NULL pointer. */
 
                         m->cached_rindex_part = part;
                         m->cached_rindex_part_begin = begin;