fix the warnings

@@ -304,7 +304,7 @@ int SrsKafkaCache::flush(SrsKafkaPartition* partition, int key, SrsKafkaPartitio
     }
     
     // remove the messages from cache.
-    if (pc->size() == nb_msgs) {
+    if ((int)pc->size() == nb_msgs) {
         pc->clear();
     } else {
         pc->erase(pc->begin(), pc->begin() + nb_msgs);

@@ -723,7 +723,7 @@ int SrsServer::acquire_pid_file()
     
     // write the pid
     string pid = srs_int2str(getpid());
-    if (write(fd, pid.c_str(), pid.length()) != pid.length()) {
+    if (write(fd, pid.c_str(), pid.length()) != (int)pid.length()) {
         ret = ERROR_SYSTEM_PID_WRITE_FILE;
         srs_error("write our pid error! pid=%s file=%s ret=%#x", pid.c_str(), pid_file.c_str(), ret);
         return ret;