compiler: Fix issue where untouched data would be petrified upon rebu…

…ilding

devito/ir/equations/algorithms.py

@@ -279,11 +279,16 @@ def _(d, mapper, rebuilt, sregistry):
         functions = rebuilt[d.functions]
     except KeyError:
         fkwargs.update({'name': sregistry.make_name(prefix=d.functions.name),
-                        'function': None,
-                        'allocator': DataReference(d.functions._data)})
+                        'function': None})
+
+        # Data in MultiSubDimension function may not have been touched at this point,
+        # in which case do not use an allocator, as there is nothing to allocate, and
+        # doing so will petrify the `_data` attribute as None.
+        if d.functions._data is not None:
+            fkwargs.update({'allocator': DataReference(d.functions._data)})
 
         rebuilt[d.functions] = functions = d.functions._rebuild(**fkwargs)
 
     kwargs['functions'] = functions
 
-    mapper[d] = d._rebuild(**kwargs)
+    mapper[d] = d._rebuild(**kwargs)