sparse-revlog: fix delta validity computation When considering the validity of a delta with sparse-revlog, we check the size of the largest read. To do so, we use some regular logic with the extra delta information. Some of this logic was not handling this extra delta properly, confusing it with "nullrev". This confusion with nullrev lead to wrong results for this computation but preventing a crash. Changeset 781b2720d2ac on default revealed this error, crashing. This changeset fixes the logic on stable so that the computation is correct (and the crash is averted). The fix is made on stable as this will impact 4.7 clients interacting with sparse-revlog repositories (eg: created by later version).