Possible typo/unclear comment in joinpath.c

In joinpath.c three times we reference "extra_lateral_rels" (with
underscores like it's a field), but as far as I can tell that's not a
field anywhere in the source code, and looking at the code that
follows it seems like it should be referencing "lateral_relids" (and
the "extra" is really "extra [in relation to relids]").

Assuming that interpretation is correct, I'd attached a patch to
change all three occurrences to "extra lateral_relids" to reduce
confusion.

Thanks,
James