If the cache needs to grow to make room to insert volatile files like
deb files into the cache we were remapping null-pointers making them
non-null-pointers in the process causing trouble later on.
Only the current Releasefile pointer can currently legally be a
nullpointer as volatile files have no release file they belong to, but
for safety the pointer to the current Packages file is equally guarded.
The option APT::Cache-Start can be used to workaround this problem.
Reported-By: Mattia Rizzolo on IRC