Stop calling Atom person constructs w/ URI invalid

If I am reading the code right, then the conditional I have patched is supposed to do the same thing as the one in the block that parses `email` elements.

Namely, the `email` variable is initialised to `NULL`, so if it already had an allocation when an `<email>` element is encountered, then there must have been another `<email>` element in the person construct before, and this second `<email>` constitutes an error per RFC 4287.

RFC 4287 specifies the same arity for the `<uri>` element, i.e. zero or one `<uri>` elements. The code also initialises the `uri` variable to `NULL` just like the `email` variable. But the block that parses `<uri>` elements checks if the `uri` variable is **not** `NULL`. This check will *always* be true when the *first* `<uri>` element is encountered, so *any* person construct that contains a URI will be declared invalid.

As far as I can tell, this is a bug.

I noticed this because I recently finally upgraded from 1.8.x, and in many of my feeds, the author metadata suddenly stopped showing up.