Fix bug in inline discussion post creation

Commit 994ccd1 contained a bug that caused new posts created via the
courseware interface to not get posted to the correct topic. They would
appear to work but would actually be posted to the "undefined" topic.
This has now been fixed.

JIRA: FOR-154
Merge pull request #4350 from edx/gprice/for-154

Fix bug in inline discussion post creation