Bug 1081403 - [email] two searches that result in the same number of matches can lead to empty search list

Clears oldListSize when clearDisplay is called.

As for risk to impact on other list displays besides search, clearDisplay is only called in two places right now, both in message_list:

* showFolder(), where the data source changes anyway, so no bad interactions there.
* showSearch(), which is the pathway that exhibited the bug.

So I believe this change to be safe from unintended consequences, and given its use for really display purposes only, makes sense to do.