Don't add write errors into bytecounts

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

apps/s_server.c

diff --git a/apps/s_server.c b/apps/s_server.c
index 61d13f3..e46376d 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -2290,8 +2290,10 @@
                     ret = 1;
                     goto err;
                 }
-                l += k;
-                i -= k;
+                if (k > 0) {
+                    l += k;
+                    i -= k;
+                }
                 if (i <= 0)
                     break;
             }