mlx4_core: Fix possible bad free in mlx4_buf_free()

When mlx4_buf_free() is called from the error path of
mlx4_buf_alloc(), it may be passed a buffer structure that does not
have all pages filled in.  Add a check for NULL to mlx4_buf_free() so
we avoid passing NULL to dma_free_coherent() (which will crash).

Signed-off-by: Ali Ayoub <ali@mellanox.co.il>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>