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

Signed-off-by: Aditya Deshpande <aditya.deshpande@arm.com>
diff --git a/programs/pkey/key_app.c b/programs/pkey/key_app.c
index 4d60299..a757cb3 100644
--- a/programs/pkey/key_app.c
+++ b/programs/pkey/key_app.c
@@ -95,7 +95,7 @@
     mbedtls_mpi_init(&D); mbedtls_mpi_init(&E); mbedtls_mpi_init(&DP);
     mbedtls_mpi_init(&DQ); mbedtls_mpi_init(&QP);
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto cleanup;