Lift the handshake driving in write_bytes up to SSL_write.

This removes one use of in_handshake and consolidates some DTLS and TLS
code.

Change-Id: Ibbdd38360a983dabfb7b18c7bd59cb5e316b2adb
Reviewed-on: https://boringssl-review.googlesource.com/7435
Reviewed-by: David Benjamin <davidben@google.com>

ssl/ssl_lib.c

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index d1ea2fe..f9ce9db 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -628,6 +628,19 @@
   }
 
   ERR_clear_system_error();
+
+  /* If necessary, complete the handshake implicitly. */
+  if (SSL_in_init(ssl) && !SSL_in_false_start(ssl)) {
+    int ret = SSL_do_handshake(ssl);
+    if (ret < 0) {
+      return ret;
+    }
+    if (ret == 0) {
+      OPENSSL_PUT_ERROR(SSL, SSL_R_SSL_HANDSHAKE_FAILURE);
+      return -1;
+    }
+  }
+
   return ssl->method->ssl_write_app_data(ssl, buf, num);
 }