asn1_string_to_utf8 test: appease coverity

Check for ASN_STRING_to_UTF8() failure before checking it matches our
expectations. This should convey clearly that test->want_len is never
negative.

CID 377011

Diagnosed by jsing

Author: tb

src/regress/lib/libcrypto/asn1/asn1_string_to_utf8.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: asn1_string_to_utf8.c,v 1.1 2022/05/16 20:53:20 tb Exp $ */
+/*	$OpenBSD: asn1_string_to_utf8.c,v 1.2 2022/11/23 08:51:05 tb Exp $ */
 /*
  * Copyright (c) 2022 Theo Buehler <tb@openbsd.org>
  *
@@ -86,12 +86,18 @@ asn1_string_to_utf8_test(const struct asn1_string_to_utf8_test_case *test)
 		goto err;
 	}
 
-	if ((ret = ASN1_STRING_to_UTF8(&out, str)) != test->want_len) {
+	if ((ret = ASN1_STRING_to_UTF8(&out, str)) < 0) {
 		warnx("ASN1_STRING_to_UTF8 failed: got %d, want %d", ret,
 		    test->want_len);
 		goto err;
 	}
 
+	if (ret != test->want_len) {
+		warnx("ASN1_STRING_to_UTF8: got %d, want %d", ret,
+		    test->want_len);
+		goto err;
+	}
+
 	if (memcmp(out, test->want, test->want_len) != 0) {
 		warnx("memcmp failed");
 		goto err;