Memory leak due to thinko in PL/Python error handling

I have realized that there's an oversight in my recent commit
c6f7f11d8, which so far from preventing edge-case leaks as intended,
actually introduces a memory leak in the normal error-catching path.
The leakage is easy to see if you extract the
catch_python_unique_violation() test case from plpython_error.sql
and run it in a loop, like

pl_regression=# do $$
begin
for i in 1..1000000 loop
perform catch_python_unique_violation();
end loop; end$$;

The backend's VIRT size as reported by top(1) will grow steadily.

The problem is that I did not think through the fact that
PyObject_GetAttrString() acquires a refcount on the returned object,
so that after advancing to the next frame object with

tb = PyObject_GetAttrString(tb, "tb_next");

we have an extra refcount on the new "tb" object, and there's no logic
that will get rid of that. The loop that I introduced with an eye to
cleaning things up:

/* Must release all the objects in the traceback stack */
while (tb != NULL && tb != Py_None)
{
PyObject *tb_prev = tb;

tb = PyObject_GetAttrString(tb, "tb_next");
Py_DECREF(tb_prev);
}

isn't right either, since it likewise doesn't account for the
extra refcount added by PyObject_GetAttrString.

The correct fix, I believe, is as attached. If we avoid collecting
extra refcounts during PLy_traceback(), then PLy_elog_impl() can go
back to simply doing Py_XDECREF(tb) on the first frame. Any
additional frames will go away when the previous frame object is
cleaned up and drops its refcount.

I also added a couple of comments explaining why PLy_elog_impl()
doesn't try to free the strings acquired from PLy_get_spi_error_data()
or PLy_get_error_data(). That's because I got here by looking at a
Coverity complaint about how those strings might get leaked. They
are not leaked, but in testing that I discovered this other leak.

regards, tom lane