Fix use of position variables with plus/merge

Fix use of position variables with plus/merge

If a selection contained a position variable (e.g., 'com of ...') that
was used more than once, and at least one of those uses was with
plus/merge, there were out-of-bounds memory writes.  This was caused by
the internal position structure not getting fully initialized.
Incomplete initialization happens in all contexts with such variables,
but only plus/merge (and possibly permute) actually use the values that
remained uninitialized, which caused them to incorrectly compute the
amount of memory required to store the result.

Fixes part of #2086.

Change-Id: I016e796db268a11d557309935c02cbd1bc79a83c