fix intialization of thread private variable

[aoymt]

Fix OpenMP bug in upvpotdiag_b.f90:

• thread-private variable iondb was passed to veps_b uninitialized.
• veps_b reads the second argument before updating it, so with OpenMP enabled each thread used undefined iondb and produced wrong/non-deterministic gradients.
• Fix: set iondb=0.d0 immediately before the call to veps_b inside the parallel region.
• The issue is detected by testad when OpenMP is enabled, including with OMP_NUM_THREADS=1.