Fix fdb_open fail in custom_compare_variable_test()

- Since the minimum chunk size of ForestDB is 4 bytes, dividing
sizeof(void*) by 2 causes error on 32-bit machine.

Change-Id: I3b54ededd4bac2c9c5e0975284b7f00fe5ec1577
Merge "Fix fdb_open fail in custom_compare_variable_test()"