Mnemonic test case DurablePersonNGTest fails after repeating runnings.

Mnemonic test case DurablePersonNGTest fails after running the same case for multiple times. This issue was firstly observed during creation of a runall.sh script file to run all test examples at same time. The issue was later reproduced with a shell script set to run this test case for 20 times.

Error message summary as follows:
"SIGSEGA (0xb) at pc=0x00007f2ae579d42c, pid=7157, tid=139822447503104"
"Problematic frame:
[pmallocallocator5618444669143453855.so+0x342c] allocate_blockfrag+0x3c
Failed to write core dump, Core dumps have been disabled."

Please note this issue in the example category for test case DurablePersonNGTest.

Thanks!

Yanhui