[MPSQ] fix a memleak in butil::ObjectPoolAllocator #1346

# to view logs

Job	Run time
clang-compile-with-bazel	9m 27s
clang-compile-with-bazel	9m 27s
clang-compile-with-bazel-all-options	11m 13s
clang-compile-with-bazel-all-options	11m 13s
clang-compile-with-boringssl	11m 35s
clang-compile-with-boringssl	11m 35s
clang-compile-with-cmake	2m 45s
clang-compile-with-cmake	2m 45s
clang-compile-with-cmake-all-options	4m 51s
clang-compile-with-cmake-all-options	4m 51s
clang-compile-with-make	3m 4s
clang-compile-with-make	3m 4s
clang-compile-with-make-all-options	5m 34s
clang-compile-with-make-all-options	5m 34s
clang-unittest	15m 46s
clang-unittest	15m 46s
gcc-compile-with-bazel	10m 26s
gcc-compile-with-bazel	10m 26s
gcc-compile-with-bazel-all-options	12m 57s
gcc-compile-with-bazel-all-options	12m 57s
gcc-compile-with-boringssl	12m 39s
gcc-compile-with-boringssl	12m 45s
gcc-compile-with-cmake	3m 15s
gcc-compile-with-cmake	3m 15s
gcc-compile-with-cmake-all-options	5m 33s
gcc-compile-with-cmake-all-options	5m 33s
gcc-compile-with-make-all-options	5m 8s
gcc-compile-with-make-all-options	5m 8s
gcc-compile-with-make	2m 51s
gcc-compile-with-make	2m 51s
	3h 54m 14s

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