Unsafe i32 to usize conversion from network data

[sumanthakur1976]

This patch fixes the problem with uncheckd data
conversion read from the network. Currently, code
reads data from the network, converts it to
i32, and then blindly converts it to usize (so -1
gets converted to -1_usize and causes panic in
debug build and huge memory use in release build).
This could easily become a security vulnerability.

The fix replaces all as usize casts to try_into
style casts.

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