Always release filesys_lock when checking if file is valid in process_execute

src/userprog/process.c

@@ -87,9 +87,10 @@ process_execute (const char *cmd)
           might be necessary. */
   lock_acquire (&filesys_lock);
   /* Validates that the current file to be executed is a valid file */
-  if (filesys_open (file_name) == NULL)
-      return TID_ERROR;
+  bool valid_file = filesys_open (file_name) != NULL;
   lock_release (&filesys_lock);
+  if (!valid_file)
+    return TID_ERROR;
 
   /* Create a new thread to execute the command, by initializing
      it running the function 'start_process' with the appropriate