SQLExecute returns 0 rows with BoolsAsChar=1

If you SQLPrepare a query, and bind a VARCHAR parameter with column_size
of exactly 5, and you have BoolsAsChar=1 (which is the default), the
subsequent SQLExecute will return 0 rows.

With those conditions, HowToPrepareBeforeExec returns shouldExec, which
means that the query is sent to the server to be parse, in a Parse
message. PGAPI_Execute() calls HowToPrepareBeforeExec, and calls
prepareParameters(), which sends the Parse message, and reads the
resulting ParameterDescription messages from the server. It also creates
a new QResultClass object, and stores it as the statement's result. Now,
when PGAPI_Execute() proceeds to execute the statement for real, it
doesn't clear the dummy result set created by prepareParameters(), but
just appends the real result set to the statement object's chain of
results. When SQLExecute() returns, the first result set (the one that
the application cares about) is the dummy one, with 0 rows.

As a quick fix, but which I believe is quite safe and correct anyway, is
to always clear any existing result sets in PGAPI_Execute() before
actually executing the query. Normally, there shouldn't be any result
sets, so clearing is harmless. In this buggy corner, case it clears the
dummy result set, fixing the problem.

Attached is a patch to fix that. As usual, this is also present in my
github repository at https://github.com/hlinnaka/psqlodbc/, as well as a
test case in the master-with-testcases branch.

- Heikki