[Python-mode] python-mode/gud bug

[Gustav Hållberg]

Hello!

This bug was found in python-mode version 4.6.18.2 (and a bunch of earlier
versions).

If you load python-mode and then start debugging a non-Python program using
gdb, the py-pdbtrack-overlay-arrow function will incorrectly set
overlay-arrow-position to nil (and therefore delete the overlay arrow), despite
not "owning" it.

To fix this, I'd suggest using some variable to keep track of whether the
overlay arrow is owned by python-mode, and only set overlay-arrow-position to
nil when that is the case. Very brief code inspection suggests that
py-pdbtrack-is-tracking-p can be used for this.

- Gustav