Don't send emails for non-existent table

When an admin logs in to upgrade to 1.7.1 and further from a version
pervious to 1.7.1, the system will attempt to clear password reset tokens
from the config table, which hasn't been upgraded yet to the namespaced
version from 1.7.1
Merge pull request #696 from greezybacon/issue/squelch-pre-upgrade

Don't send emails for non-existent table

Reviewed-By: Peter Rotich <peter@osticket.com>