block_cache-test: fix an ASAN failure due to mismatched alloc/free
block_cache-test: fix an ASAN failure due to mismatched alloc/free

This is a new error type caught by trunk builds of clang++ -- the test code was
using malloc (via strdup) to allocate something, and delete[] (in the cache) to
free it.

  ==18173==ERROR: AddressSanitizer: alloc-dealloc-mismatch (malloc vs operator delete []) on 0x60200000e050
      #0 0x45b2db in operator delete[](void*) _asan_rtl_
      #1 0x47b0c5 in kudu::cfile::BlockCache::ValueDeleter(kudu::Slice const&, void*) /home/todd/git/kudu/src/cfile/block_cache.cc:72
      #2 0x482dd9 in kudu::(anonymous namespace)::LRUCache::Unref(kudu::(anonymous namespace)::LRUHandle*) /home/todd/git/kudu/src/util/cache.cc:197
      #3 0x488a0b in ~LRUCache /home/todd/git/kudu/src/util/cache.cc:187
      #4 0x487ede in ~ShardedLRUCache /home/todd/git/kudu/src/util/cache.cc:299
      #5 0x48016b in ~ShardedLRUCache /home/todd/git/kudu/src/util/cache.cc:299
      #6 0x47444a in base::DefaultDeleter<kudu::Cache>::operator()(kudu::Cache*) const /home/todd/git/kudu/src/gutil/gscoped_ptr.h:145
      #7 0x474249 in ~gscoped_ptr_impl /home/todd/git/kudu/src/gutil/gscoped_ptr.h:228
      #8 0x4740cc in ~gscoped_ptr_impl /home/todd/git/kudu/src/gutil/gscoped_ptr.h:224
      #9 0x473fbc in ~gscoped_ptr /home/todd/git/kudu/src/gutil/gscoped_ptr.h:318
      #10 0x473dac in ~gscoped_ptr /home/todd/git/kudu/src/gutil/gscoped_ptr.h:318
      #11 0x473b37 in ~BlockCache /home/todd/git/kudu/src/cfile/block_cache.h:17
      #12 0x47080c in ~BlockCache /home/todd/git/kudu/src/cfile/block_cache.h:17
      #13 0x46ebb3 in kudu::cfile::TestBlockCache_TestBasics_Test::TestBody() /home/todd/git/kudu/src/cfile/block_cache-test.cc:41
      #14 0x4df61c in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ??:0
      #15 0x4da57d in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) ??:0
      #16 0x4c6dec in testing::Test::Run() ??:0
      #17 0x4c7561 in testing::TestInfo::Run() ??:0
      #18 0x4c7b33 in testing::TestCase::Run() ??:0
      #19 0x4cca5f in testing::internal::UnitTestImpl::RunAllTests() ??:0
      #20 0x4e0b10 in bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) ??:0
      #21 0x4db48b in bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) ??:0
      #22 0x4cb7b1 in testing::UnitTest::Run() ??:0
      #23 0x4e9699 in main /home/todd/git/kudu/src/util/test_main.cc:15
      #24 0x7f6012dbc76c in __libc_start_main /build/buildd/eglibc-2.15/csu/libc-start.c:226
  0x60200000e050 is located 0 bytes inside of 12-byte region [0x60200000e050,0x60200000e05c)
  allocated by thread T0 here:
      #0 0x45ac4b in __interceptor_malloc _asan_rtl_
      #1 0x7f6012e23d81 in __GI___strdup /build/buildd/eglibc-2.15/string/strdup.c:43
  ==18173==HINT: if you don't care about these warnings you may set ASAN_OPTIONS=alloc_dealloc_mismatch=0
  ==18173==ABORTING

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