Add missing state to DTLS state machine.

This was a mistake from when we added async CertificateVerify support.
No test because the final state of each write state is semi-unreachable
due to the buffer BIO that gets installed on each handshake.

Change-Id: I0180926522113c8b1ca58b8c9c6dc37fb0dd8083
Reviewed-on: https://boringssl-review.googlesource.com/6412
Reviewed-by: Adam Langley <agl@google.com>

ssl/d1_clnt.c

@@ -345,6 +345,7 @@ int dtls1_connect(SSL *s) {
 
       case SSL3_ST_CW_CERT_VRFY_A:
       case SSL3_ST_CW_CERT_VRFY_B:
+      case SSL3_ST_CW_CERT_VRFY_C:
         dtls1_start_timer(s);
         ret = ssl3_send_cert_verify(s);
         if (ret <= 0) {