Evas filters: fix potential memory leak

Unlikely use case trying to delete a source when there are no
sources yet.

@fix

src/lib/evas/canvas/evas_object_text.c

@@ -2397,6 +2397,7 @@ _evas_text_filter_source_set(Eo *eo_obj, Evas_Text_Data *o, const char *name, Ev
 
    if (!o->cur.filter.sources)
      {
+        if (!source) return;
         o->cur.filter.sources = eina_hash_string_small_new
               (EINA_FREE_CB(_filter_source_hash_free_cb));
      }