Fix a memory leak on win32 socket->event map.

This would lose some memory every time an event_base was freed on win32.

Found by Dimitre Piskyulev.

evmap.c

@@ -148,6 +148,7 @@ void evmap_io_clear(struct event_io_map *ctx)
 		next = HT_NEXT_RMV(event_io_map, ctx, ent);
 		mm_free(this);
 	}
+	HT_CLEAR(event_io_map, ctx); /* remove all storage held by the ctx. */
 }
 #endif