fix: Network.Pcap.statistics to throw on non-live Pcap handles

[vlm]

Avoid making Haskell treat C int as long (Haskell's Int). Without this patch:

Prelude Network.Pcap> openOffline "test/empty.pcap" >>= statistics 
Statistics {statReceived = 129485792, statDropped = 1, statIfaceDropped = 129485816}
Prelude Network.Pcap> openOffline "test/empty.pcap" >>= statistics 
Statistics {statReceived = 129683784, statDropped = 1, statIfaceDropped = 115781272}
Prelude Network.Pcap> 

(returns different garbage every time it is called).

With this patch: > cabal repl --ghc-options="-lpcap"

Prelude Network.Pcap> openOffline "test/empty.pcap" >>= statistics
*** Exception: user error (Statistics aren't available from savefiles)
Prelude Network.Pcap> 

Bryan, thank you for maintaining this package!