Fix premature use of optional argument in mpas_get_stream_filename

If no optional `ierr` argument is provided in calling the subroutine
`mpas_get_stream_filename()` the model segfaults by trying to assign to
the nonexistent argument. Additionally, `ierr` is set to zero to
initialize, but as this module uses its own error codes it should
instead use `MPAS_STREAM_MGR_NOERR`.

This change checks if `ierr` is present, and if so initializes it to
`MPAS_STREAM_MGR_NOERR`.

Author: islas

src/framework/mpas_stream_manager.F

@@ -4073,7 +4073,9 @@ subroutine mpas_get_stream_filename(manager, streamID, when, blockID, filename,
         type (MPAS_TimeInterval_type) :: filename_interval
         type (MPAS_Time_type) :: now_time
 
-        ierr = 0
+        if ( present(ierr) ) then
+           ierr = MPAS_STREAM_MGR_NOERR
+        end if
 
         if ( present(blockID) ) then
            blockID_local = blockID