set_buffer_lengths_to_zero is unused if bufferevents is not enabled on Windows
if bufferevents are not enabled, on Windows, set_buffer_lengths_to_zero is defined but unused. This causes a gcc warning (or error, if -Werror is set). The attached patch only enables set_buffer_lengths_to_zero if on Windows and bufferevents are enabled.
Applied on b5a306e82c684bdd30b832fdfd9e2b55c06b54ae