lib: repair condition inversion in contexts_pool_insert

If a client disconnects, e.g. because gromox-http was stopped
for too long in a debugger, a log message may occur:

```
contexts_pool: failed to modify event in epoll: No such file or directory (T1), No such file or directory (T2)
contexts_pool: error in context queue! b_waiting mismatch in thread_work_func context: 0xabc
```

Fixes: gromox-1.32-15-ge32abdc21 last hunk

Author: jengelh

lib/contexts_pool.cpp

@@ -409,7 +409,7 @@ void contexts_pool_insert(schedule_context *pcontext, sctx_status tpraw)
 			}
 		} else if (g_poll_ctx.mod(pcontext, false) != 0) {
 			int se = errno;
-			if (errno == ENOENT && g_poll_ctx.mod(pcontext, true) != 0) {
+			if (errno == ENOENT && g_poll_ctx.mod(pcontext, true) == 0) {
 				/* sometimes, fd will be removed by scanning
 				thread because of timeout, add it back
 				into epoll queue again */