Call BIO_flush when sending a DTLS 1.3 ACK

I don't expect any DTLS BIO internally buffers and requires a flush, but
we call it for send_flight, so we ought to be consistent.

Bug: 42290594
Change-Id: If3e390c7ad295da0966767362bd78ee574d718e7
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/73428
Reviewed-by: Nick Harper <nharper@chromium.org>
Commit-Queue: David Benjamin <davidben@google.com>

ssl/d1_both.cc

diff --git a/ssl/d1_both.cc b/ssl/d1_both.cc
index a40710d..838c21b 100644
--- a/ssl/d1_both.cc
+++ b/ssl/d1_both.cc
@@ -1091,6 +1091,11 @@
     return bio_ret;
   }
 
+  if (BIO_flush(ssl->wbio.get()) <= 0) {
+    ssl->s3->rwstate = SSL_ERROR_WANT_WRITE;
+    return -1;
+  }
+
   return 1;
 }