Free external entity parser in external_entity_param_checker()

Removes some of the memory leaks detected by AddressSanitizer
in the test suite (see Issue #23)

expat/tests/runtests.c

@@ -2047,6 +2047,7 @@ external_entity_param_checker(XML_Parser parser,
         return XML_STATUS_ERROR;
     }
     handler_data = parser;
+    XML_ParserFree(ext_parser);
     return XML_STATUS_OK;
 }