Add missing assertions.

Add missing assertions.

For both QTestLog::addFail() and QTestLog::addIgnoreMessage(), passing
a null message does not make sense and is therefore an error that
should be brought to the developer's attention.

Change-Id: Ib09ad90b70d74f7432c08708db8a70dee008cce4
Reviewed-on: http://codereview.qt.nokia.com/3470
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>