Avoid re-using output variables in new ecpg test case.

Avoid re-using output variables in new ecpg test case.

The buildfarm thinks this leads to memory stomps, though annoyingly
I can't duplicate that here.  The existing code in strings.pgc is
doing something that doesn't seem to be sanctioned at all really
by the documentation, but I'm disinclined to try to make that nicer
right now.  Let's just declare some more output variables in hopes
of working around it.