heap_tuple_needs_freeze false positive

Hi,

I noticed that heap_tuple_needs_freeze might return true in cases where
the Xmax is leftover junk from somebody who set HEAP_XMAX_INVALID in the
far past without resetting the Xmax value itself to Invalid.  I think
this is incorrect usage; the rule, I think, is that one shouldn't even
read Xmax at all unless HEAP_XMAX_INVALID is reset.

This might cause unnecessary acquisitions of the cleanup lock, if a
tuple is deemed freezable when in fact it isn't.

Suggested patch attached.  I'd backpatch this as far as it applies
cleanly.

-- 
Álvaro Herrera <alvherre@alvh.no-ip.org>