Fix for bug #11378 (https://gna.org/bugs/?11378).
This bug was reported by Ryan Hoffman (https://gna.org/users/rydog).
The fix was to set the xh_vect variable to None rather than deleting it when the proton or heteronucleus is not found by the pdb() user function, and to also deselect that spin.
Authored by: bugman 2008-03-27
Parent: [r5234]
Child: [r5236]
