Fix index refcount in unbox_dataframe_column (#599)

Author: Ivan Butygin

sdc/hiframes/boxing.py

@@ -296,6 +296,7 @@ def codegen(context, builder, sig, args):
 
         c.pyapi.decref(series_obj)
         c.pyapi.decref(arr_obj)
+        c.context.nrt.incref(builder, df_typ.index, dataframe.index)
 
         # assign array and set unboxed flag
         dataframe.data = builder.insert_value(