fix invalid conversion error with MinGW 4.8.1 in net.cpp

- fixes src\net.cpp:1601: Error:invalid conversion from 'void*' to
  'const char*' [-fpermissive] in a setsockopt() call on Win32 that was
  found by using MinGW 4.8.1 compiler suite
Merge pull request #2826 from Diapolo/fix_mingw481

fix invalid conversion error with MinGW 4.8.1 in net.cpp