extension_get_list() will return NULL if there is no extension device.
Check for this situation.
Addresses-Coverity-ID: 638557 - Null pointer dereferences (NULL_RETURNS)
Fixes: 2d12958ee7 ("boot: Remove legacy extension board support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Kory Maincent <kory.maincent@bootlin.com>