fix: check for <=0 max dist 3d cartesian

see #626

assimilation_code/location/threed_cartesian/location_mod.f90

@@ -1046,10 +1046,21 @@ subroutine get_close_init(gc, num, maxdist, locs, maxdist_list)
 
 if ( .not. module_initialized ) call initialize_module
 
+if (maxdist <= 0.0_r8) then
+   write(msgstring, *) 'bad maxdist value ', maxdist, ' , must be > 0'
+   call error_handler(E_ERR, 'get_close_init', msgstring, source)
+endif
+
 typecount = get_num_types_of_obs()
 allocate(gc%type_to_cutoff_map(typecount)) 
 
 if (present(maxdist_list)) then
+
+   if (any(maxdist_list <= 0.0_r8)) then
+      write(msgstring, *) 'bad maxdist_list value ', maxdist_list, ' , must be > 0'
+      call error_handler(E_ERR, 'get_close_init', msgstring, source)
+   endif
+
    if (size(maxdist_list) .ne. typecount) then
       write(msgstring,'(A,I8,A,I8)')'maxdist_list len must equal number of specific types, ', &
                                     size(maxdist_list), ' /= ', typecount