Fixes an issue where the CollectPageLatchFields can cause an access violation (AV) under certain timing conditions when the latch_suspend_end XEvent is turned on. The underlying reason is that this method assumes the Buffer is stable under the latch, which isn't true if the Buffer isn't in the Hashtable anymore. This hotfix avoids the AV by checking whether the Buffer is still in the Hashtable before accessing its underlying fields.
