Fixed sporadically occuring floating invalid error in qpw_to_nmt

The error occured in line 172 and could be tracked back to a missing
initialization of rho in stden.

optional/stden.f90

@@ -96,6 +96,7 @@
           ALLOCATE ( rht(vacuum%nmzd,2,DIMENSION%jspd),vbar(2,atoms%ntypd),sigm(vacuum%nmzd) )
           ALLOCATE ( rhoss(DIMENSION%msh,DIMENSION%jspd) )
           enpara%enmix=1.0
+          rho = 0.0
           IF (mpi%irank == 0) THEN
 
              !--->    if sigma is not 0.0, then divide this charge among all atoms