Fix an ancient typo that prevented the detection of conflicting fields when

Fix an ancient typo that prevented the detection of conflicting fields when
interval input "invalid" was specified together with other fields.  Spotted
by Neil Conway with the help of a clang warning.  Although this has been
wrong since the interval code was written more than 10 years ago, it doesn't
affect anything beyond which error message you get for a wrong input, so not
worth back-patching very far.