MinGW 4.8.1 Compilation Fix

fix invalid conversion error with MinGW 4.8.1 in net.cpp

Bitcoin compilation bug also fixed at https://github.com/bitcoin/bitcoin/commit/6c6255edb54bed780f0879c906dccf6cfa98b4db
Merge pull request #30 from Delinquency/patch-1

MinGW 4.8.1 Compilation Fix