_views_prepare_handler() doesn't set defaults correctly

When the 'group', 'title' or 'help' slots are missing from a definition, _views_prepare_handler() consults the field-level, then the table-level. But the code that checks the table-level is faulty (the conditional is structured badly). The result is that a slot that actually exists in a definition may get overriden by the table's one.

(Note: the faulty code was looking for a mysterious 'table' array in the definition itself. I assume this is a bug: the only definition that seems to have a 'table' slot is a join's one, but there it's a simple string, not an array, and moreover a join handler isn't at all prepared by _views_prepare_handler.)