fix misaligned-pointer-use in StlVectorTest
fix misaligned-pointer-use in StlVectorTest

Summary:
Exposed by UBSAN's misaligned-pointer-use:
```lang=bash
> folly/test/stl_tests/StlVectorTest.cpp:544:12: runtime error: upcast of misaligned address 0x0000feebdaed for type 'DataTracker<false>', which requires 8 byte alignment
> 0x0000feebdaed: note: pointer points here
> <memory cannot be printed>
>     #0 0x6c9b93 in DataTracker<false>::~DataTracker() folly/test/stl_tests/StlVectorTest.cpp:544
>     #1 0x6c94fe in Data<0u, 0ul>::~Data() folly/test/stl_tests/StlVectorTest.cpp:605
>     #2 0xcc5f93 in void test_iteratorInsertionN2<folly::fbvector<Data<0u, 0ul>, std::allocator<Data<0u, 0ul> > > >(std::integral_constant<bool, true>) folly/test/stl_tests/StlVectorTest.cpp:2422
>     #3 0x5c9bac in void test_iteratorInsertionN3<folly::fbvector<Data<0u, 0ul>, std::allocator<Data<0u, 0ul> > > >() folly/test/stl_tests/StlVectorTest.cpp:2422
>     #4 0x5c67bd in FBVector_iteratorInsertionN_Test::TestBody() folly/test/stl_tests/StlVectorTest.cpp:2422
>     #5 0x7f9c4e5316c9 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:2464
>     #6 0x7f9c4e530dcc in testing::Test::Run() xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:2480
>     #7 0x7f9c4e5350a0 in testing::TestInfo::Run() xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:2662
>     #8 0x7f9c4e539408 in testing::TestCase::Run() xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:2780
>     #9 0x7f9c4e55a672 in testing::internal::UnitTestImpl::RunAllTests() xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:4655
>     #10 0x7f9c4e5595de in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:2464
>     #11 0x7f9c4e558b72 in testing::UnitTest::Run() xplat/third-party/gmock/googletest-1.8.0/googletest/src/gtest.cc:4263
>     #12 0x6591f3 in RUN_ALL_TESTS() buck-out/cells/fbsource/gen/xplat/third-party/gmock/gtest_headers#header-mode-symlink-tree-with-header-map,headers/gtest/gtest.h:2247
>     #13 0x659139 in main folly/test/stl_tests/StlVectorTest.cpp:3074
>     #14 0x7f9c4c75b1a5 in __libc_start_main /home/engshare/third-party2/glibc/2.26/src/glibc-2.26/csu/libc-start.c:308:16
>     #15 0x4de029 in _start /home/engshare/third-party2/glibc/2.26/src/glibc-2.26/sysdeps/x86_64/start.S:120
>
> SUMMARY: UndefinedBehaviorSanitizer: misaligned-pointer-use folly/test/stl_tests/StlVectorTest.cpp:544:12
```

Make the crafted pointer address aligned.

Reviewed By: Gownta

Differential Revision: D19166262

fbshipit-source-id: f766656d031079350838598b135275bdbc047642

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