The string-input-visitor happily accepts NaN and infinities when parsing
numbers (doubles). They shouldn't. Fix that.

Also, add two test cases, testing if "NaN" and "inf" is properly
rejected.
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20181121164421.20780-4-david@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>