tools/decode_simple: always call process_frame(NULL) at the end

Currently this would not be done if max_frames is triggered.

Makes no difference in either of the tools currently using
decode_simple, but may be important in future tools.

tools/decode_simple.c

@@ -73,7 +73,7 @@ int ds_run(DecodeContext *dc)
     while (ret >= 0) {
         ret = av_read_frame(dc->demuxer, dc->pkt);
         if (ret < 0)
-            goto flush;
+            break;
         if (dc->pkt->stream_index != dc->stream->index) {
             av_packet_unref(dc->pkt);
             continue;
@@ -91,10 +91,9 @@ int ds_run(DecodeContext *dc)
             fprintf(stderr, "Error decoding: %d\n", ret);
             return ret;
         } else if (ret > 0)
-            return 0;
+            goto finish;
     }
 
-flush:
     avcodec_send_packet(dc->decoder, NULL);
     ret = decode_read(dc, 1);
     if (ret < 0) {
@@ -102,7 +101,8 @@ flush:
         return ret;
     }
 
-    return 0;
+finish:
+    return dc->process_frame(dc, NULL);
 }
 
 void ds_free(DecodeContext *dc)