Nullify pointers on repeated calls to write_trajectory()

- When calling write_trajectory() a second time, the declaration
    type(buffer), pointer :: obuffer_io=>null()
  does not nullify the pointer. Added explicit nullification
  in the code in addition.
- No answer changes.

icebergs_io.F90

@@ -829,14 +829,17 @@ subroutine write_trajectory(trajectory)
 integer :: stderrunit
 
 !I/O vars
-type(xyt), pointer :: traj4io=>NULL()
+type(xyt), pointer :: traj4io=>null()
 integer :: ntrajs_sent_io,ntrajs_rcvd_io
 integer :: from_pe,np
 type(buffer), pointer :: obuffer_io=>null(), ibuffer_io=>null()
 logical :: io_is_in_append_mode
 
   ! Get the stderr unit number
   stderrunit=stderr()
+  traj4io=>null()
+  obuffer_io=>null()
+  ibuffer_io=>null()
 
   !Assemble the list of trajectories from all pes in this I/O tile