fix(arm): check the presence of the policy check function

Change-Id: I7bb0dad232eac6c8084873713af22562853a4c21
Signed-off-by: Jimmy Brisson <jimmy.brisson@arm.com>
diff --git a/plat/arm/common/arm_io_storage.c b/plat/arm/common/arm_io_storage.c
index 19ee1b0..5a3d27a 100644
--- a/plat/arm/common/arm_io_storage.c
+++ b/plat/arm/common/arm_io_storage.c
@@ -124,6 +124,7 @@
 	const struct plat_io_policy *policy;
 
 	policy = FCONF_GET_PROPERTY(arm, io_policies, image_id);
+	assert(policy->check != NULL);
 	result = policy->check(policy->image_spec);
 	if (result == 0) {
 		*image_spec = policy->image_spec;