Avoid a "constant variable guards dead code" warning from coverity

Fixes CID 752028

src/common/compat_libevent.c

@@ -210,6 +210,9 @@ tor_libevent_initialize(tor_libevent_cfg *torcfg)
     } else {
       using_iocp_bufferevents = 0;
     }
+#elif defined(__COVERITY__)
+    /* Avoid a 'dead code' warning below. */
+    using_threads = ! torcfg->disable_iocp;
 #endif
 
     if (!using_threads) {