PR2039: Fix NULL pointer access

In case rtems_bdbuf_read() returns an error status, the block device
buffer pointer will be set to NULL.  In RFS the chain node of the block
device buffer will be used for RFS purposes.  We must not do this after
an erroneous read.

cpukit/libfs/src/rfs/rtems-rfs-buffer.c

@@ -168,8 +168,6 @@ rtems_rfs_buffer_handle_request (rtems_rfs_file_system*   fs,
   {
     rc = rtems_rfs_buffer_io_request (fs, block, read, &handle->buffer);
 
-    rtems_chain_set_off_chain (rtems_rfs_buffer_link(handle));
-
     if (rc > 0)
     {
       if (rtems_rfs_trace (RTEMS_RFS_TRACE_BUFFER_HANDLE_REQUEST))
@@ -177,6 +175,8 @@ rtems_rfs_buffer_handle_request (rtems_rfs_file_system*   fs,
                 block, read ? "read" : "get", rc, strerror (rc));
       return rc;
     }
+
+    rtems_chain_set_off_chain (rtems_rfs_buffer_link(handle));
   }
 
   /*