It happens if the copy command errors out before assigning
dispatcherState. Initialize the dispatcherState as NULL to fix it,
palloc0() to avoid future new member issues.

5X has no such problem.

```
(gdb) c
Continuing.
Detaching after fork from child process 25843.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000aa04dd in getCdbCopyPrimaryGang (c=0x23d4150) at cdbcopy.c:44
44              return (Gang *)linitial(c->dispatcherState->allocatedGangs);
(gdb) bt
\#0  0x0000000000aa04dd in getCdbCopyPrimaryGang (c=0x23d4150) at cdbcopy.c:44
\#1  0x0000000000aa12d8 in cdbCopyEndAndFetchRejectNum (c=0x23d4150, total_rows_completed=0x0, abort_msg=0xd0c8f8 "aborting COPY in QE due to error in QD") at cdbcopy.c:642
\#...
(gdb) p c->dispatcherState
$1 = (struct CdbDispatcherState *) 0x100000000
```