The "socket" variable is not available on Windows even though
the --socket option can be used to specify the pipe name for
local connections that use a named pipe.

The solution is to ensure that the variable is always defined.


mysql-test/r/windows.result:
  Add test case result for Bug#45498
mysql-test/t/windows.test:
  Add test case for Bug#45498
sql/set_var.cc:
  socket variable must always be present.