gdb/python: fix an unlikely memory leak

gdb/python: fix an unlikely memory leak

I noticed a possible memory leak in gdbpy_create_ptid_object, in
py-infthread.c.  We create a Tuple, and hold the reference in a
'PyObject*' local.

If we then fail to create any of the tuple contents we perform an
early exit, returning nullptr, this will leak the Tuple object.

Currently, we create the Tuple as the first action in the function,
but we don't really need the tuple until the end of the function.

In this commit I have:

  1. Moved creation of the Tuple until the end of the function, just
     before we need it.

  2. Stored the Tuple reference in a gdbpy_ref<>.  This is not
     strictly needed any more, but is (I think) good practice as
     future changes to the function will not need to worry about
     releasing the Tuple object.

  3. Taken the opportunity to replace a NULL with nullptr in this
     function.

  4. Inlined the local variable declarations to the point of first
     use.

There should be no user visible changes after this commit.

No tests as I have no idea how to make gdb_py_object_from_longest (and
friends) fail, and so trigger the memory leak.  I suspect we'd never
actually see this leak in the real world, but it doesn't hurt to clean
these things up.

Approved-By: Simon Marchi <simon.marchi@efficios.com>