Use a single exit_status instead of two

src/userprog/syscall.c

@@ -131,7 +131,7 @@ syscall_exit (int status)
 {
   /* Sets exit_status of the thread to status. thread_exit () will call
      process_exit () if user programs are allowed. */
-  thread_current ()->exit_status = status;
+  thread_current ()->result->exit_status = status;
   thread_exit ();
 }
 
@@ -271,7 +271,7 @@ syscall_read (int fd, void *buffer, unsigned size)
     {
       /* Reading from the console. */
       char *write_buffer = buffer;
-      for (int i = 0; i < size; i++)
+      for (unsigned i = 0; i < size; i++)
         write_buffer[i] = input_getc ();
 
       /* In case of console, read is always (eventually) successful. So return