segmentation fault if host unknown

This issue has imported from below and all of patches written by mindw0rm.
https://code.google.com/p/mysql-udf-http/issues/detail?id=3

> Reported by mindw...@web.de, Aug 22, 2013
> When specifying an unknown host, a segmentation fault occurs:
> mysql> select http_get('http://example.invalid/');
> ERROR 2013 (HY000): Lost connection to MySQL server during query
> mysql> 
> 
> The mysql error log says "mysqld got signal 11 ;"
> 
> 
> After some debugging I found that in that case result->res ist still NULL after curl_easy_perform. The error is caused by 
> strcopy(res->result, "")