Fix return value on impossible error condition

This codepath is impossible because CBB_add_utf8 will never write more
than 4 bytes, much less 6.

Change-Id: Ibc97ea1518186bba128a6853d7f039f097a2184e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/93568
Commit-Queue: Xiangfei Ding <xfding@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Reviewed-by: Xiangfei Ding <xfding@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>

crypto/asn1/a_strex.cc

diff --git a/crypto/asn1/a_strex.cc b/crypto/asn1/a_strex.cc
index bddcfcb..6aecec5 100644
--- a/crypto/asn1/a_strex.cc
+++ b/crypto/asn1/a_strex.cc
@@ -135,7 +135,7 @@
       CBB_init_fixed(&utf8_cbb, utf8_buf, sizeof(utf8_buf));
       if (!CBB_add_utf8(&utf8_cbb, c)) {
         OPENSSL_PUT_ERROR(ASN1, ERR_R_INTERNAL_ERROR);
-        return 1;
+        return -1;
       }
       size_t utf8_len = CBB_len(&utf8_cbb);
       for (size_t i = 0; i < utf8_len; i++) {