Allow PSK binder mismatches in fuzzer mode.

BUG=112

Change-Id: I88ef17e32e33b091ff1e27b7950f88e1d48f9278
Reviewed-on: https://boringssl-review.googlesource.com/12239
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/tls13_enc.c

diff --git a/ssl/tls13_enc.c b/ssl/tls13_enc.c
index c846d9c..d87d8a6 100644
--- a/ssl/tls13_enc.c
+++ b/ssl/tls13_enc.c
@@ -438,8 +438,12 @@
     return 0;
   }
 
-  if (CBS_len(&binder) != hash_len ||
-      CRYPTO_memcmp(CBS_data(&binder), verify_data, hash_len) != 0) {
+  int binder_ok = CBS_len(&binder) == hash_len &&
+                  CRYPTO_memcmp(CBS_data(&binder), verify_data, hash_len) == 0;
+#if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
+  binder_ok = 1;
+#endif
+  if (!binder_ok) {
     OPENSSL_PUT_ERROR(SSL, SSL_R_DIGEST_CHECK_FAILED);
     return 0;
   }