fix invalid conversion error with MinGW 4.8.1 in net.cpp
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

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