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/cert_req.c b/programs/x509/cert_req.c
index 1126e60..8ef5932 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.c
@@ -159,7 +159,7 @@
     mbedtls_ctr_drbg_init(&ctr_drbg);
     memset(buf, 0, sizeof(buf));
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;