There was a double free issue which was fixed in 772bca3f, add test
case for it.

    commit 772bca3f
    Author: Pengzhou Tang <ptang@pivotal.io>
    Date:   Tue Mar 20 22:06:50 2018 -0400

        Fix SIGSEGV issue when freeing gangs

        Previously, to avoid the leak of the gang if someone terminates
        the query in the middle of gang creation, we added a global pointer
        named CurrentGangCreating so the partially created gang can also be
        destroyed at the end of the transaction. However, the memory context
        named GangContext where CurrentGangCreating was created may be reset
        before CurrentGangCreating is actually destroyed and a SIGSEGV may
        occur. So this commit makes sure that CurrentGangCreating is destroyed
        ahead of other created gangs and the reset of GangContext.
Co-Authored-by: NJialun Du <jdu@pivotal.io>
Co-Authored-by: NNing Yu <nyu@pivotal.io>