staging/lustre/ptlrpc: Removes potential null dereference

The lustre_msg_buf method could return NULL. Subsequent code didn't
check if it's null before using it. This patch adds two checks.

Signed-off-by: Lidza Louina <lidza.louina@oracle.com>
Acked-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>