IB/ucm: Initialize sgid request GID attribute pointer

sgid_attr is uninitialized on the stack, initialize it to NULL.

Fixes: 39839107 ("IB/cm: Replace members of sa_path_rec with 'struct sgid_attr *'")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Yossi Itigin <yosefe@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>