xfsprogs: unhandled error check in libxfs_trans_read_buf

xfsprogs: unhandled error check in libxfs_trans_read_buf

libxfs_trans_read_buf() is used in both mkfs.xfs & xfs_repair.
During stability testing we found some time occur pagefault in
mkfs.xfs, code inspection shows that if libxfs_readbuf() fails then
occurs a page fault in xfs_buf_item_init() called in
libxfs_trans_read_buf().

mkfs.xfs: unhandled page fault (11) at 0x00000070, code 0x017

Added NULL check and errno handling.

Signed-off-by: Ajeet Yadav <ajeet.yadav.77@gmail.com>
Signed-off-by: Alex Elder <aelder@sgi.com>