specifier: avoid leaking memory on allocation error

==8036==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 64 byte(s) in 1 object(s) allocated from:
    #0 0x4a10bc in __interceptor_realloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:85:3
    #1 0x4deef1 in realloc (/build/fuzz-unit-file+0x4deef1)
    #2 0x7ffa35abfe23 in greedy_realloc /work/build/../../src/systemd/src/basic/alloc-util.c:70:13
    #3 0x7ffa35aefad2 in parse_env_file_internal /work/build/../../src/systemd/src/basic/env-file.c:127:38
    #4 0x7ffa35af08a6 in parse_env_file_fdv /work/build/../../src/systemd/src/basic/env-file.c:374:13
    #5 0x7ffa35b6391e in parse_extension_release_atv /work/build/../../src/systemd/src/basic/os-util.c:323:16
    #6 0x7ffa35b63c8a in parse_extension_release_sentinel /work/build/../../src/systemd/src/basic/os-util.c:360:13
    #7 0x7ffa35a5e3f5 in parse_os_release_specifier /work/build/../../src/systemd/src/shared/specifier.c:292:13
    #8 0x7ffa35a5e3f5 in specifier_os_id /work/build/../../src/systemd/src/shared/specifier.c:303:16
    #9 0x7ffa35a5c7f5 in specifier_printf /work/build/../../src/systemd/src/shared/specifier.c:70:45
    #10 0x7ffa3690b279 in unit_full_printf_full /work/build/../../src/systemd/src/core/unit-printf.c:264:16
    #11 0x7ffa367de795 in config_parse_bus_name /work/build/../../src/systemd/src/core/load-fragment.c:2401:13
    #12 0x7ffa358fe5ec in next_assignment /work/build/../../src/systemd/src/shared/conf-parser.c:151:24
    #13 0x7ffa358fe5ec in parse_line /work/build/../../src/systemd/src/shared/conf-parser.c:257:16
    #14 0x7ffa358fd653 in config_parse /work/build/../../src/systemd/src/shared/conf-parser.c:400:21
    systemd#15 0x4de828 in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/core/fuzz-unit-file.c:72:16
    systemd#16 0x4df208 in NaloFuzzerTestOneInput (/build/fuzz-unit-file+0x4df208)
    systemd#17 0x4fe213 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    systemd#18 0x4fd9fa 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
    systemd#19 0x4ff0c9 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    systemd#20 0x4ffd95 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
    systemd#21 0x4ef0ff in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    systemd#22 0x4ef9c8 in LLVMFuzzerRunDriver /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:925:10
    systemd#23 0x4df485 in main (/build/fuzz-unit-file+0x4df485)
    systemd#24 0x7ffa35232082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: 1878e6b475720c7c51969e69ab2d276fae6d1dee)

DEDUP_TOKEN: __interceptor_realloc--realloc--greedy_realloc
SUMMARY: AddressSanitizer: 64 byte(s) leaked in 1 allocation(s).

Found by Nallocfuzz.

src/shared/specifier.c

@@ -284,7 +284,7 @@ int specifier_architecture(char specifier, const void *data, const char *root, c
  * installation. */
 
 static int parse_os_release_specifier(const char *root, const char *id, char **ret) {
-        char *v = NULL;
+        _cleanup_free_ char *v = NULL;
         int r;
 
         assert(ret);
@@ -293,7 +293,7 @@ static int parse_os_release_specifier(const char *root, const char *id, char **r
         if (r >= 0)
                 /* parse_os_release() calls parse_env_file() which only sets the return value for
                  * entries found. Let's make sure we set the return value in all cases. */
-                *ret = v;
+                *ret = TAKE_PTR(v);
 
         /* Translate error for missing os-release file to EUNATCH. */
         return r == -ENOENT ? -EUNATCH : r;