Death, etc. of President-elect before oath of office
Section 136 · 1999 Constitution (2004 edition)
What Constitution says
(1) If a person duly elected as President dies before taking and subscribing the Oath of Allegiance and oath of office, or is for any reason whatsoever unable to be sworn in, the person elected with him as Vice-President shall be sworn in as President and he shall nominate a new Vice-President who shall be appointed by the President with the approval by a simple majority of the National Assembly at a joint sitting. (2) Where the persons duly elected as President and Vice President die or are unable for any reason whatsoever to assume office before the inauguration of the National Assembly, the Independent National Electoral Commission shall immediately conduct an election for a President and the Vice-President.
This is the 1999 Constitution as published in 2004. Later amendments aren't included yet, so a section may have changed since.
Original source
Official Gazette · CFRN 1999 official PDF · printed page 69. Original document
Release: draft-c8d50d87fae9. Source scope, review and corrections