Since network requests are, for the most part, asynchronous, there's a
chance a request only comes through after the request sender has already
been deleted.
This adds a global (read static) hash table relating models for the mod
downloader to their status (true = alive, false = destroyed). It is a
bit of a hack, but I couldn't come up with a better way of doing this.
To reproduce the issue before this commit: scroll really quickly through
CF mods, to trigger network requests for their versions and description.
Then, in the middle of it close the mod downloader. Sometimes this will
create a crash.
Signed-off-by: flow <flowlnlnln@gmail.com>