Handle 0-byte reads and writes correctly.

asio/include/asio/ssl/detail/impl/engine.ipp

diff --git a/asio/include/asio/ssl/detail/impl/engine.ipp b/asio/include/asio/ssl/detail/impl/engine.ipp
index 1333506..9241e8d 100644
--- a/asio/include/asio/ssl/detail/impl/engine.ipp
+++ b/asio/include/asio/ssl/detail/impl/engine.ipp
@@ -125,6 +125,12 @@
 engine::want engine::write(const asio::const_buffer& data,
     asio::error_code& ec, std::size_t& bytes_transferred)
 {
+  if (asio::buffer_size(data) == 0)
+  {
+    ec = asio::error_code();
+    return engine::want_nothing;
+  }
+
   return perform(&engine::do_write,
       const_cast<void*>(asio::buffer_cast<const void*>(data)),
       asio::buffer_size(data), ec, &bytes_transferred);
@@ -133,6 +139,12 @@
 engine::want engine::read(const asio::mutable_buffer& data,
     asio::error_code& ec, std::size_t& bytes_transferred)
 {
+  if (asio::buffer_size(data) == 0)
+  {
+    ec = asio::error_code();
+    return engine::want_nothing;
+  }
+
   return perform(&engine::do_read,
       asio::buffer_cast<void*>(data),
       asio::buffer_size(data), ec, &bytes_transferred);