Fix MSVC warning in sample programs

The warning was caused because of conversions from size_t to int, which
can cause data loss. The files affected are:
* ssl_client2.c
* ssl_server2.c
* ssl_mail_client.c
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 1856cc4..af3b0d3 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -1267,7 +1267,7 @@
 
     len = polarssl_snprintf( (char *) buf, sizeof(buf) - 1, GET_REQUEST,
                     opt.request_page );
-    tail_len = strlen( GET_REQUEST_END );
+    tail_len = (int) strlen( GET_REQUEST_END );
 
     /* Add padding to GET request to reach opt.request_size in length */
     if( opt.request_size != DFL_REQUEST_SIZE &&