Fixed MSAN warning by freeing the arena in ~InlinedArena().

This fixes an MSAN warning of the form:

```
WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7fc964f2a597 in arena_findroot third_party/upb/upb/mem/arena.c:64:3
    #1 0x7fc964f2a597 in upb_Arena_Free third_party/upb/upb/mem/arena.c:211:7
    #2 0x7fc9d2af0028 in std::__msan::unique_ptr<upb_Arena, void (*)(upb_Arena*)>::~unique_ptr() third_party/crosstool/v18/stable/toolchain/bin/../include/c++/v1/__memory/unique_ptr.h:277:7
    #3 0x7fc9d2aef7a2 in ~Arena third_party/upb/upb/upb.hpp:70:7
    #4 0x7fc9d2aef7a2 in ~InlinedArena third_party/upb/upb/upb.hpp:97:7
    #5 0x7fc9d2aef7a2 in Cpp_InlinedArena2_Test::TestBody() third_party/upb/upb/test/test_cpp.cc:187:1
    #6 0x7fc97da78a57 in testing::Test::Run() third_party/googletest/googletest/src/gtest.cc:2695:5
    #7 0x7fc97da7a3e8 in testing::TestInfo::Run() third_party/googletest/googletest/src/gtest.cc:2844:11
    #8 0x7fc97da7b897 in testing::TestSuite::Run() third_party/googletest/googletest/src/gtest.cc:3003:30
    #9 0x7fc97daa5136 in testing::internal::UnitTestImpl::RunAllTests() third_party/googletest/googletest/src/gtest.cc:5899:44
    #10 0x7fc97daa455c in HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> third_party/googletest/googletest/src/gtest.cc
    #11 0x7fc97daa455c in testing::UnitTest::Run() third_party/googletest/googletest/src/gtest.cc:5464:10
    #12 0x562a7fb876f0 in RUN_ALL_TESTS third_party/googletest/googletest/include/gtest/gtest.h:2329:73
    #13 0x562a7fb876f0 in main testing/base/internal/gunit_main.cc:86:10
    #14 0x7fc9ba9b7632 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x61632) (BuildId: 280088eab084c30a3992a9bce5c35b44)
    #15 0x562a7fafdbe9 in _start /build/work/ab393f4ac612f9027aae6b1a7226027ba2a2/google3/blaze-out/k8-opt/bin/third_party/grte/v5_src/grte-scratch/BUILD/src/csu/../sysdeps/x86_64/start.S:120

  Member fields were destroyed
    #0 0x562a7fb0b13d in __sanitizer_dtor_callback_fields third_party/llvm/llvm-project/compiler-rt/lib/msan/msan_interceptors.cpp:960:5
    #1 0x7fc9d2aef79a in ~InlinedArena third_party/upb/upb/upb.hpp:105:8
    #2 0x7fc9d2aef79a in ~InlinedArena third_party/upb/upb/upb.hpp:97:7
    #3 0x7fc9d2aef79a in Cpp_InlinedArena2_Test::TestBody() third_party/upb/upb/test/test_cpp.cc:187:1
    #4 0x7fc97da78a57 in testing::Test::Run() third_party/googletest/googletest/src/gtest.cc:2695:5
    #5 0x7fc97da7a3e8 in testing::TestInfo::Run() third_party/googletest/googletest/src/gtest.cc:2844:11
    #6 0x7fc97da7b897 in testing::TestSuite::Run() third_party/googletest/googletest/src/gtest.cc:3003:30
    #7 0x7fc97daa5136 in testing::internal::UnitTestImpl::RunAllTests() third_party/googletest/googletest/src/gtest.cc:5899:44
    #8 0x7fc97daa455c in HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool> third_party/googletest/googletest/src/gtest.cc
    #9 0x7fc97daa455c in testing::UnitTest::Run() third_party/googletest/googletest/src/gtest.cc:5464:10
    #10 0x562a7fb876f0 in RUN_ALL_TESTS third_party/googletest/googletest/include/gtest/gtest.h:2329:73
    #11 0x562a7fb876f0 in main testing/base/internal/gunit_main.cc:86:10
    #12 0x7fc9ba9b7632 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x61632) (BuildId: 280088eab084c30a3992a9bce5c35b44)
    #13 0x562a7fafdbe9 in _start /build/work/ab393f4ac612f9027aae6b1a7226027ba2a2/google3/blaze-out/k8-opt/bin/third_party/grte/v5_src/grte-scratch/BUILD/src/csu/../sysdeps/x86_64/start.S:120
```

PiperOrigin-RevId: 511849224

upb/test/test_cpp.cc

@@ -126,6 +126,11 @@ TEST(Cpp, InlinedArena) {
   EXPECT_EQ(0, n);
 }
 
+TEST(Cpp, InlinedArena2) {
+  upb::InlinedArena<64> arena;
+  upb_Arena_Malloc(arena.ptr(), sizeof(int));
+}
+
 TEST(Cpp, Default) {
   upb::DefPool defpool;
   upb::Arena arena;

upb/upb.hpp

@@ -87,7 +87,7 @@ class Arena {
 
   void Fuse(Arena& other) { upb_Arena_Fuse(ptr(), other.ptr()); }
 
- private:
+ protected:
   std::unique_ptr<upb_Arena, decltype(&upb_Arena_Free)> ptr_;
 };
 
@@ -97,6 +97,11 @@ template <int N>
 class InlinedArena : public Arena {
  public:
   InlinedArena() : Arena(initial_block_, N) {}
+  ~InlinedArena() {
+    // Explicitly destroy the arena now so that it does not outlive
+    // initial_block_.
+    ptr_.reset();
+  }
 
  private:
   InlinedArena(const InlinedArena*) = delete;