Clear the error queue in fuzzer-mode Channel ID hooks.

Otherwise it leaves something on the error queue and confuses
SSL_get_error, should the handshake state machine fail immediately
afterwards because of a BIO-level error.

Change-Id: I2c7b5e31368b9c5b2efa324166f52972430d6074
Reviewed-on: https://boringssl-review.googlesource.com/24247
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

ssl/t1_lib.cc

@@ -3264,6 +3264,7 @@ int tls1_verify_channel_id(SSL_HANDSHAKE *hs, const SSLMessage &msg) {
   int sig_ok = ECDSA_do_verify(digest, digest_len, sig.get(), key.get());
 #if defined(BORINGSSL_UNSAFE_FUZZER_MODE)
   sig_ok = 1;
+  ERR_clear_error();
 #endif
   if (!sig_ok) {
     OPENSSL_PUT_ERROR(SSL, SSL_R_CHANNEL_ID_SIGNATURE_INVALID);