Fix build with ZMQ_HAVE_WINDOWS (typedef UINT_PTR fd_t; -> unsigned type)
  V547 Expression 'file_desc >= 0' is always true. Unsigned type value is always >= 0. socket_base.cpp 845
  V547 Expression 'file_desc >= 0' is always true. Unsigned type value is always >= 0. socket_base.cpp 863
  V547 Expression 'file_desc >= 0' is always true. Unsigned type value is always >= 0. socket_base.cpp 897
  http://www.viva64.com/en/d/0137/print/
Merge pull request #902 from pavel-pimenov/fix-pvs-studio-v547

Fix build with ZMQ_HAVE_WINDOWS (typedef UINT_PTR fd_t; -> unsigned type...