rust: bssl-tls: Call inherent flush when flushing through std io trait

Signed-off-by: Xiangfei Ding <xfding@google.com>
Change-Id: I4f285255134906b8a4004569071d12156a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/101412
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Reviewed-by: Rudolf Polzer <rpolzer@google.com>

diff --git a/rust/bssl-tls/src/connection/io/stdio.rs b/rust/bssl-tls/src/connection/io/stdio.rs
index 3406818..e048bcc 100644
--- a/rust/bssl-tls/src/connection/io/stdio.rs
+++ b/rust/bssl-tls/src/connection/io/stdio.rs

@@ -76,7 +76,7 @@
     }
 
     fn flush(&mut self) -> io::Result<()> {
-        Ok(())
+        translate_res_for_stdio(self.flush()).map(|_| ())
     }
 }