Prevent invalid frame durations

Prevent frame duration from being negative when
exporting movie studio project

Change-Id: I459fb243650c777c2b59d1094acc88c3a944bd81
Fix for crash if no content in DESCRIBE response

If DESCRIBE response is received with status 200 but no content,
MyHandler will still set content data for session description
parsing. This will cause NULL Pointer crash.
This fix checks whether DESCRIBE response has content before
parsing session description.

Change-Id: I114ae6fd54ce804e61718f62618ca9008425a433