[crashtracker] Limit size of files in the crashinfo

[danielsn]

          Really unrelated to this PR, but I noticed that `CrashInfo::add_file` seems to read the entire file to memory before trying to do anything with its contents.

Perhaps it may be worth adding a hard limit there, in case we get fed the wrong thing? (Insert crowdstrike broken file joke/reference?)

Originally posted by @ivoanjo in #551 (comment)