Fix for getredash#1521

The merge from getredash#1521 fixed the invalid variable error
but it fixed in in a way that introduced another error:
"dictionary changed size during iteration"
This correctly applies the fix.

redash/query_runner/elasticsearch.py

@@ -119,12 +119,13 @@ def _get_mappings(self, url):
         return mappings, error
 
     def _get_query_mappings(self, url):
-        mappings, error = self._get_mappings(url)
+        mappings_data, error = self._get_mappings(url)
         if error:
-            return mappings, error
+            return mappings_data, error
 
-        for index_name in mappings:
-            index_mappings = mappings[index_name]
+        mappings = {}
+        for index_name in mappings_data:
+            index_mappings = mappings_data[index_name]
             for m in index_mappings.get("mappings", {}):
                 for property_name in index_mappings["mappings"][m]["properties"]:
                     property_data = index_mappings["mappings"][m]["properties"][property_name]