The ast parameter cannot be null, because the only use checks for it not
to be null. However, if it would ever be re-used somewhere else, the
logic is plain wrong. Clarification by an assert makes it clear what the
intent is.

Pointed out by the clang static analyzer.

Change-Id: I2c8cba5e5847fc1f92c10021109c55ff8ccd58c4
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>