Fix mpas_stream_list_insert bug unlinking head on duplicate insert

Fix bug in mpas_stream_list_insert that could unlink the head node when a
duplicate stream was inserted. Moved nullify(stream % next) calls into the
relevant conditional blocks to ensure new streams are only linked after
passing duplicate checks. Prevents inadvertent modification of the list when
duplicate insertions occur.

Author: Andy Stokely

src/framework/mpas_stream_list.F

@@ -112,9 +112,9 @@ subroutine MPAS_stream_list_insert(list, stream, ierr) !{{{
 
         if (present(ierr)) ierr = MPAS_STREAM_LIST_NOERR
 
-        nullify(stream % next)
         if (.not. associated(list % head)) then
             list % head => stream
+            nullify(stream % next)
         else
             node => list % head
             do while (associated(node))
@@ -125,6 +125,7 @@ subroutine MPAS_stream_list_insert(list, stream, ierr) !{{{
                 end if
                 if (.not. associated(node % next)) then
                     node % next => stream
+                    nullify(stream % next)
                     exit
                 end if
                 node => node % next