Win32: event flags handling edge cases in ngx_wsarecv().

Win32: event flags handling edge cases in ngx_wsarecv().

Fixed event flags handling edge cases in ngx_wsarecv() and ngx_wsarecv_chain(),
notably to always reset rev->ready in case of errors (which wasn't the case
after ngx_socket_nread() errors), and after EOF (rev->ready was not cleared
if due to a misconfiguration a zero-sized buffer was used for reading).