[fix] google_videos engine: some results don't a thumbnail

searx/engines/google_videos.py

@@ -75,8 +75,10 @@ def response(resp):
 
         # get thumbnails
         script = str(dom.xpath('//script[contains(., "_setImagesSrc")]')[0].text)
-        id = result.xpath('.//div[@class="s"]//img/@id')[0]
-        thumbnails_data = re.findall('s=\'(.*?)(?:\\\\[a-z,1-9,\\\\]+\'|\')\;var ii=\[(?:|[\'vidthumb\d+\',]+)\'' + id,
+        ids = result.xpath('.//div[@class="s"]//img/@id')
+        if len(ids) > 0:
+            thumbnails_data = \
+                re.findall('s=\'(.*?)(?:\\\\[a-z,1-9,\\\\]+\'|\')\;var ii=\[(?:|[\'vidthumb\d+\',]+)\'' + ids[0],
                            script)
             tmp = []
             if len(thumbnails_data) != 0: