Fix for MDEV-13191. Assert for !is_set() when doing LOAD DATA

This could happen when the client connection dies while sending a progress
report packet.
Fixed by not raising any errors when sending progress packets.

sql/sql_class.cc

@@ -4084,6 +4084,10 @@ extern "C" enum thd_kill_levels thd_kill_level(const MYSQL_THD thd)
    however not more often than global.progress_report_time.
    If global.progress_report_time is 0, then don't send progress reports, but
    check every second if the value has changed
+
+  We clear any errors that we get from sending the progress packet to
+  the client as we don't want to set an error without the caller knowing
+  about it.
 */
 
 static void thd_send_progress(THD *thd)
@@ -4100,8 +4104,12 @@ static void thd_send_progress(THD *thd)
     thd->progress.next_report_time= (report_time +
                                      seconds_to_next * 1000000000ULL);
     if (global_system_variables.progress_report_time &&
-        thd->variables.progress_report_time)
+        thd->variables.progress_report_time && !thd->is_error())
+    {
       net_send_progress_packet(thd);
+      if (thd->is_error())
+        thd->clear_error();
+    }
   }
 }