CRM-15210 fix fatal on merge exports of non primary addresses

CRM-15210 fix fatal on merge exports of non primary addresses
The relationship check is preventing the fields from being transformed
to the correct field names in contacts not-to-be-merged
The possibility of the object not existing is adequately handled further down in the
function. The check was added during svn-days but after 4.2.4
I no longer seem to be able to access what commit introduced it