PVS-Studio warning V512 for libevent

void
win32_dealloc(struct event_base *_base)
{
  struct win32op *win32op = _base->evbase;
  ....
  memset(win32op, 0, sizeof(win32op));
  ....
}

This suspicious code was found in libevent project by PVS-Studio static code analyzer.
Warning message is:
V512 A call of the 'memset' function will lead to underflow of the buffer 'win32op'. win32select.c 374

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/