bugfix - plpgsql - statement counter is incremented 2x for FOR stmt

Hi

When I fixed one plpgsql_check issue, I found another plpgsql issue. Now we
have field nstatements that hold a number of plpgsql statements in
function. Unfortunately I made an error when I wrote this functionality and
for FOR statements, this counter is incremented 2x. Higher number than a
real number is better than a lesser number, but it can be a source of
problems too (inside plpgsql_check I iterate over 0 .. nstatements stmtid,
and due this bug I had a problem with missing statements).

Attached patch is pretty simple:

Regards

Pavel