processFile() accessed files_[id - 1] without validating id.
When id == 0 (from a crafted network packet), id - 1 = -1 caused
out-of-bounds access. Added bounds check: id must be in range
[1, files_.size()]. Stops receive and logs error on invalid id.
apply some code analyzer recommendations
ICU flag now check if libicu exists
prepare for more accurate growth of containers (limited PoT, then constantly increase size)