mpm_winnt: clear OVERLAPPED structs before reuse

MSDN documentation states that

    Any unused members of [an OVERLAPPED] structure should always be
    initialized to zero before the structure is used in a function call.
    Otherwise, the function may fail and return ERROR_INVALID_PARAMETER.

Prior to this patch, the internal state left over from previous
overlapped I/O was passed into the next call. It's unclear what effect
this might have, if any. (I have not personally witnessed an
ERROR_INVALID_PARAMETER myself.)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1758310 13f79535-47bb-0310-9956-ffa450edef68