Bug#535218: apt-get / aptitude segfault when Cache-Limit is too small

[Gustaw Smolarczyk <wielkiegie@gmail.com>]

Same problem here. I've just increased APT::Cache-Limit and the 
problem's gone, but I think too that the problem must be fixed in repo 
(just changing the default) or provide users better messages in case of 
too small amount of cache (not just SEGV).

This problem is caused in amd64 by i386 repository. It doubles required 
cache, so I think we should also double default cache limit.