fixes initial condition search procedure

TurTLE/DNS.py

@@ -881,6 +881,7 @@ class DNS(_code):
                         for cp in range(source_current_checkpoint+1)]
                 src_file = None
                 for check_file in source_checkpoints:
+                    if os.path.exists(check_file):
                         f0 = h5py.File(check_file, 'r')
                         if ('{0}'.format(opt.src_iteration)
                            in f0[checkpoint_field + '/complex'].keys()):