Fix bugs in example programs: change argc == 0 to argc < 2

Signed-off-by: Aditya Deshpande <aditya.deshpande@arm.com>
diff --git a/programs/x509/req_app.c b/programs/x509/req_app.c
index 8cfe4a4..83e2546 100644
--- a/programs/x509/req_app.c
+++ b/programs/x509/req_app.c
@@ -70,7 +70,7 @@
      */
     mbedtls_x509_csr_init(&csr);
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;