SgmlLinkExtractor - fix for parsing <area> tag with Unicode present

`current_link` wasn't cleaned during reset and might have been reused for next pass of `extract_links`. Easiest way to see it is to try to 'extract_links' from response containing `<area>` tag or un-closed `<a>` tag two times in a row.

Additionally text field of link was converted to Unicode so when it was reused `handle_data` tried to concatenate Unicode and string and if string contained any characters not from ascii range it died with exception `UnicodeError`.