fix stress calculation with independent particles

Currently (a56657f9 on main branch), the stress tensor calculation with independent particles is failing.

**************************** FATAL ERROR *****************************
*** Fatal Error (description follows)
*--------------------------------------------------------------------
* From node =    0
*--------------------------------------------------------------------
* Node 0:
*  Assertion "ubound(dx, dim = 1) >= n1"
*  failed in line 508 of file "math/lalg_basic_blas_inc.F90".
*--------------------------------------------------------------------
* Stack: > main/run.F90.run > main/ground_state.F90.ground_state_run > main/ground_state.F90.ground_state_run_legacy > scf/electrons_ground_state.F90.ground_state_run_legacy > scf/scf.F90.scf_run > electrons/stress.F90.stress_calculate > electrons/stress.F90.stress_from_pseudo_loc > math/lalg_basic_blas_inc.F90.copy_1_2
**********************************************************************