xfs: avoid double-free in xfs_attr_node_addname

xfs_attr_node_addname()'s error handling tests whether it
should free "state" in the out: error handling label:

out:
        if (state)
                xfs_da_state_free(state);

but an earlier free doesn't set state to NULL afterwards; this
could lead to a double free.  Fix it by setting state to NULL
after it's freed.

This was found by Coverity.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>