capability-util: avoid false-positive use-of-uninitialized-value error

fuzzers randomly fail with the following:
```
==172==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7f41169cb39b in update_argv /work/build/../../src/systemd/src/basic/argv-util.c:96:13
    #1 0x7f41169cb39b in rename_process /work/build/../../src/systemd/src/basic/argv-util.c:210:16
    #2 0x7f4116b6824e in safe_fork_full /work/build/../../src/systemd/src/basic/process-util.c:1516:21
    #3 0x7f4116bffa36 in safe_fork /work/build/../../src/systemd/src/basic/process-util.h:191:16
    #4 0x7f4116bffa36 in parse_timestamp /work/build/../../src/systemd/src/basic/time-util.c:1047:13
    #5 0x4a61e6 in LLVMFuzzerTestOneInput /work/build/../../src/systemd/src/fuzz/fuzz-time-util.c:16:16
    #6 0x4c4a13 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
    #7 0x4c41fa 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
    #8 0x4c58c9 in fuzzer::Fuzzer::MutateAndTestOne() /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:757:19
    #9 0x4c6595 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
    #10 0x4b58ff in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:912:6
    #11 0x4def52 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
    #12 0x7f4115ea3082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082) (BuildId: e678fe54a5d2c2092f8e47eb0b33105e380f7340)
    #13 0x41f5ad in _start (build-out/fuzz-time-util+0x41f5ad)

DEDUP_TOKEN: update_argv--rename_process--safe_fork_full
  Uninitialized value was created by an allocation of 'fv' in the stack frame of function 'have_effective_cap'
    #0 0x7f41169d3540 in have_effective_cap /work/build/../../src/systemd/src/basic/capability-util.c:21
```

Author: yuwata

src/basic/capability-util.c

@@ -20,7 +20,8 @@
 
 int have_effective_cap(int value) {
         _cleanup_cap_free_ cap_t cap = NULL;
-        cap_flag_value_t fv;
+        cap_flag_value_t fv = CAP_CLEAR; /* To avoid false-positive use-of-uninitialized-value error reported
+                                          * by fuzzers. */
 
         cap = cap_get_proc();
         if (!cap)