Fix multiple bugs in wignerseitz.f90

[yurivict]

The patch fixes three bugs in wignerseitz.f90:

1. Unallocated private allocatable (tridx): Per OpenMP spec §2.15.3.3, private copies of allocatables start unallocated. The original code allocated tridx before the parallel region then listed it as private, causing undefined behavior when get_pairs tried to use the unallocated array. Fixed by allocating tridx inside the parallel region (same pattern used correctly in eeq.f90).
2. Race condition on self%nimg_max: Multiple threads wrote self%nimg_max = max(nimg, self%nimg_max) simultaneously without synchronization. Fixed with a local variable and reduction(max:nimg_max).
3. Missing termination in get_pairs do-loop: When all img translations happen to be equidistant (e.g., in high-symmetry crystals), the loop would exhaust all mask entries, then minloc with an all-false mask returns pos=0 (undefined per Fortran standard, gfortran returns 0), causing dist(0) out-of-bounds. Fixed with if (iws >= img) exit guard.

The original symptom was the crash in the pbc testcase.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 30.68%. Comparing base (282626e) to head (dab7451).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #61       +/-   ##
===========================================
- Coverage   51.57%   30.68%   -20.90%     
===========================================
  Files          18        2       -16     
  Lines        1648      176     -1472     
  Branches      746       72      -674     
===========================================
- Hits          850       54      -796     
+ Misses        390       85      -305     
+ Partials      408       37      -371     

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