[DFS] Add fd checking in dfs_file_close API.

components/dfs/src/dfs_file.c

@@ -136,6 +136,9 @@ int dfs_file_close(struct dfs_fd *fd)
 {
     int result = 0;
 
+    if (fd == RT_NULL)
+        return -DFS_STATUS_ENXIO;
+
     if (fd != RT_NULL && fd->fs->ops->close != RT_NULL)
         result = fd->fs->ops->close(fd);
 
@@ -568,6 +571,7 @@ void rm(const char *filename)
     }
 }
 FINSH_FUNCTION_EXPORT(rm, remove files or directories);
+
 void cat(const char* filename)
 {
     rt_uint32_t length;