Fix uninitialized PruneFreezeResult in pruneheap and vacuumlazy

Hi Hackers,

While reviewing Melanie's patch [1], I found this bug where presult is not
initialized. Let me explain the logic.

In the first place:
```
static int
lazy_scan_prune(LVRelState *vacrel,
Buffer buf,
BlockNumber blkno,
Page page,
Buffer vmbuffer,
bool all_visible_according_to_vm,
bool *has_lpdead_items,
bool *vm_page_frozen)
{
Relation rel = vacrel->rel;
PruneFreezeResult presult; <== here presult is not initialized

heap_page_prune_and_freeze(&params,
&presult, <== uninitialized presult is passed
into heap_page_prune_and_freeze
&vacrel->offnum,
&vacrel->NewRelfrozenXid, &vacrel->NewRelminMxid);
```

Then in heap_page_prune_and_freeze():
```
void
heap_page_prune_and_freeze(PruneFreezeParams *params,
PruneFreezeResult *presult,
OffsetNumber *off_loc,
TransactionId *new_relfrozen_xid,
MultiXactId *new_relmin_mxid)
{
Buffer buffer = params->buffer;
Page page = BufferGetPage(buffer);
PruneState prstate;
bool do_freeze;
bool do_prune;
bool do_hint_prune;
bool did_tuple_hint_fpi;
int64 fpi_before = pgWalUsage.wal_fpi;

/* Initialize prstate */
prune_freeze_setup(params,
new_relfrozen_xid, new_relmin_mxid,
presult, &prstate); <== immediately pass uninitialized presult
to prune_freeze_setup
```

Then in prune_freeze_setup():
```
static void
prune_freeze_setup(PruneFreezeParams *params,
TransactionId *new_relfrozen_xid,
MultiXactId *new_relmin_mxid,
const PruneFreezeResult *presult, <== presult is a const pointer,
so prune_freeze_setup won’t update its content
PruneState *prstate)
{
prstate->deadoffsets = (OffsetNumber *) presult->deadoffsets;
<== presult->deadoffsets could be a random value
}
```

Attached is a simple fix by just initializing presult in the first place
with {0}.

[1] https://postgr.es/m/CAAKRu_bvQiesumRhgvbcym1T9Z9=
ddGfUbi-dSNxLRc6JvL1-w(at)mail(dot)gmail(dot)com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/