MDEV-30664 mysqldump output is truncated on Windows

Flush stdout on finalizing of mysqldump/mysqlbinlog output
to avoid truncation.

The same patch has been applied to the mysqltest.cc code with
  commit 34ff714b
  Author: Magnus Svensson <msvensson@mysql.com>
  Date:   Fri Nov 14 11:06:56 2008 +0100
    WL#4189 Make mysqltest flush log file at close if logfile is stdout

but not to mysqldump.c/mysqlbinlog.cc

client/mysqlbinlog.cc

@@ -3224,8 +3224,13 @@ int main(int argc, char** argv)
 
   if (tmpdir.list)
     free_tmpdir(&tmpdir);
-  if (result_file && result_file != stdout)
+  if (result_file)
+  {
+    if (result_file != stdout)
       my_fclose(result_file, MYF(0));
+    else
+      fflush(result_file);
+  }
   cleanup();
   /* We cannot free DBUG, it is used in global destructors after exit(). */
   my_end(my_end_arg | MY_DONT_FREE_DBUG);

client/mysqldump.c

@@ -1794,8 +1794,13 @@ static FILE* open_sql_file_for_table(const char* table, int flags)
 
 static void free_resources()
 {
-  if (md_result_file && md_result_file != stdout)
+  if (md_result_file)
+  {
+    if (md_result_file != stdout)
       my_fclose(md_result_file, MYF(0));
+    else
+      fflush(md_result_file);
+  }
   if (get_table_name_result)
     mysql_free_result(get_table_name_result);
   if (routine_res)