Allow exchanges=0 in ssl_server2

Useful for testing with defensics with no data exchange
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index be9d07b..330abbe 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -869,7 +869,7 @@
         else if( strcmp( p, "exchanges" ) == 0 )
         {
             opt.exchanges = atoi( q );
-            if( opt.exchanges < 1 )
+            if( opt.exchanges < 0 )
                 goto usage;
         }
         else if( strcmp( p, "min_version" ) == 0 )
@@ -1740,6 +1740,9 @@
     }
 #endif /* POLARSSL_X509_CRT_PARSE_C */
 
+    if( opt.exchanges == 0 )
+        goto close_notify;
+
     exchanges = opt.exchanges;
 data_exchange:
     /*