using closed handle in libs/zbxexec/execute.c zbx_execute()

libs/zbxexec/execute.c zbx_execute() code, which is supposed to wait for launched process finish, is accessing previously closed handle in WaitForSingleObject() call.
it results in sporadical false execute timeouts.

also MSDN recommends calling SetHandleInformation() for read pipe handle for reasons unknown, and I trust them in that.

patch for 1.8.7 is attached