CDRIVER-756: Check for both 0 and -1 as maybe-failure-or-maybe-retry codes

Author: bjori

src/mongoc/mongoc-stream-tls.c

@@ -500,7 +500,7 @@ _mongoc_stream_tls_write (mongoc_stream_tls_t *tls,
 
    ret = BIO_write (tls->bio, buf, buf_len);
 
-   if (ret < 0) {
+   if (ret <= 0) {
       return ret;
    }