Fix tupdesc dangling pointer segfault in HashAgg
This problem manifests itself with HashAgg on the top of
DynamicIndexScan node and can cause a segmentation fault.
1. A HashAgg node initializes a tuple descriptor for its hash
slot using a reference from input tuples (coming from
DynamicIndexScan through a Sequence node).
2. At the end of every partition index scan in DynamicIndexScan
we unlink and free unused memory chunks and reset partition's
memory context. It causes a total destruction of all objects in
the context including partition index tuple descriptor used in a
HashAgg node.
As a result we get a dangling pointer in HashAgg on switching to
a new index partition during DynamicIndexScan that can cause a
segfault.
