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

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

programs/ssl/ssl_mail_client.c

@@ -370,7 +370,7 @@ int main(int argc, char *argv[])
     mbedtls_pk_init(&pkey);
     mbedtls_ctr_drbg_init(&ctr_drbg);
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);