xfs: Check the return value of xfs_buf_read() for NULL

Check the return value of xfs_buf_read() for NULL and return ENOMEM
if it is NULL.  This is necessary in a few spots to avoid subsequent
code blindly dereferencing the null buffer pointer.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Alex Elder <aelder@sgi.com>