OmxEncoder: set s->of to nullptr after close (#19814)

old-commit-hash: a5be02c2c7

selfdrive/loggerd/omx_encoder.cc

@@ -574,6 +574,7 @@ void OmxEncoder::encoder_close() {
       avformat_free_context(this->ofmt_ctx);
     } else {
       fclose(this->of);
+      this->of = nullptr;
     }
     unlink(this->lock_path);
   }