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

Signed-off-by: Aditya Deshpande <aditya.deshpande@arm.com>

programs/x509/cert_req.c

@@ -159,7 +159,7 @@ int main(int argc, char *argv[])
     mbedtls_ctr_drbg_init(&ctr_drbg);
     memset(buf, 0, sizeof(buf));
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;