unicode: add missing check for an error return from utf8lookup()

Signed-off-by: NTheodore Ts'o <tytso@mit.edu>
Cc: Gabriel Krisman Bertazi <krisman@collabora.com>

fs/unicode/utf8-norm.c

@@ -714,6 +714,8 @@ int utf8byte(struct utf8cursor *u8c)
 			}
 
 			leaf = utf8lookup(u8c->data, u8c->hangul, u8c->s);
+			if (!leaf)
+				return -1;
 			ccc = LEAF_CCC(leaf);
 		}