s4-drsuapi: Use sam_ctx consistently in dcesrv_drsuapi_DsGetNCChanges()

s4-drsuapi: Use sam_ctx consistently in dcesrv_drsuapi_DsGetNCChanges()

Trying to use bstate->sam_ctx_system by mistake can cause crashes if
non-admin users replicate. To avoid this problem we use the sam_ctx
variable, however it wasn't used consistently everywhere. Replace the
remaining references to b_state->sam_ctx to avoid potential confusion.

This change was made based on review feedback from Metze.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>