nghttpx: Fix assertion failure on TLS handshake

src/shrpx_connection.cc

@@ -146,7 +146,14 @@ int shrpx_bio_write(BIO *b, const char *buf, int len) {
   if (conn->tls.initial_handshake_done) {
     // After handshake finished, send |buf| of length |len| to the
     // socket directly.
-    assert(wbuf.rleft() == 0);
+
+    // Only when TLS session was prematurely ended before server sent
+    // all handshake message, this condition is true.  This could be
+    // alert from SSL_shutdown().  Since connection is already down,
+    // just return error.
+    if (wbuf.rleft()) {
+      return -1;
+    }
     auto nwrite = conn->write_clear(buf, len);
     if (nwrite < 0) {
       return -1;