py-pdbtrack-grub-for-buffer(): Rectified some logic errors i

py-pdbtrack-grub-for-buffer(): Rectified some logic errors i
introduced when shifting around some code, and added some redundancy
to reduce chances of hitting the wrong source code.  (This is
experimental - it will improve the accuracy, but will reduce the
ability of the user to deliberately select the buffer they want the
buffer grubbing stuff to find.  I think the accuracy improvement will
be worth it, but am not sure, so may remove this.)