Fix MockQuicTransport::Flush error handling.

BIO_flush may return a negative value, so we shouldn't cast it directly
to bool.

Change-Id: Ibdf688d1a6b4b316069e3b99a8a8b18974ee17ed
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46534
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

ssl/test/mock_quic_transport.cc

diff --git a/ssl/test/mock_quic_transport.cc b/ssl/test/mock_quic_transport.cc
index 4b8bc30..310b779 100644
--- a/ssl/test/mock_quic_transport.cc
+++ b/ssl/test/mock_quic_transport.cc
@@ -271,7 +271,7 @@
   return WriteRecord(level, SSL3_RT_APPLICATION_DATA, in, len);
 }
 
-bool MockQuicTransport::Flush() { return BIO_flush(bio_.get()); }
+bool MockQuicTransport::Flush() { return BIO_flush(bio_.get()) > 0; }
 
 bool MockQuicTransport::SendAlert(enum ssl_encryption_level_t level,
                                   uint8_t alert) {