rsa: reject images with unknown padding

Previously we would store NULL in info->padding and jump to an illegal
instruction if an unknown value for "padding" was specified in the
device tree.

Signed-off-by: Patrick Doyle <pdoyle@irobot.com>

common/image-sig.c

@@ -211,7 +211,7 @@ static int fit_image_setup_verify(struct image_sign_info *info,
 	info->required_keynode = required_keynode;
 	printf("%s:%s", algo_name, info->keyname);
 
-	if (!info->checksum || !info->crypto) {
+	if (!info->checksum || !info->crypto || !info->padding) {
 		*err_msgp = "Unknown signature algorithm";
 		return -1;
 	}