Page display issue when splitting a discussion

This was discovered in MDLQA-1191. When splitting a discussion the following error is displayed:
Coding problem: $PAGE->context was not set. You may have forgotten to call require_login() or $PAGE->set_context(). The page may not display correctly as a result
line 345 of /lib/pagelib.php: call to debugging()
line 617 of /lib/pagelib.php: call to moodle_page->magic_get_context()
line 1216 of /lib/weblib.php: call to moodle_page->__get()
line 463 of /mod/forum/post.php: call to format_string()

This patch seems to fix it (I'm not totally clear on the mechanics of $PAGE): https://github.com/mackensen/moodle/compare/master...MDL-30289