CVE-2020-1900

When unserializing an object with dynamic properties HHVM needs to
pre-reserve the full size of the dynamic property array before inserting
anything into it. Otherwise the array might resize, invalidating previously
stored references. This pre-reservation was not occurring in HHVM prior to
v4.32.3, between versions 4.33.0 and 4.56.0, 4.57.0, 4.58.0, 4.58.1,
4.59.0, 4.60.0, 4.61.0, 4.62.0.