Disallow NaN value for speed key.

The VideoBlock `handle_ajax` is allowing NaN values for speed key
and causing videos to not load. Also added a data migration to fix
the data for learners.

PROD-1148

common/lib/xmodule/xmodule/video_module/video_handlers.py

@@ -8,6 +8,7 @@ StudioViewHandlers are handlers for video descriptor instance.
 
 import json
 import logging
+import math
 
 import six
 from django.core.files.base import ContentFile
@@ -89,6 +90,11 @@ class VideoStudentViewHandlers(object):
                     if key == 'bumper_last_view_date':
                         value = now()
 
+                    if key == 'speed' and math.isnan(value):
+                        message = u"Invalid speed value {}, must be a float.".format(value)
+                        log.warning(message)
+                        return json.dumps({'success': False, 'error': message})
+
                     setattr(self, key, value)
 
                     if key == 'speed':