[PATCH] gcc warning 'expression which evaluates to zero treated as anull pointer'

Hi,
Trivial patch:
- remove a gcc warning (since commit 7a0574b5)
expression which evaluates to zero treated as a null pointer constant of
      type 'HeapTuple' (aka 'struct HeapTupleData *')

- always use "if (newtuple == NULL)" rather than mixing !newtuple and
newtuple == NULL

Regards
Didier