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_app.c b/programs/x509/cert_app.c
index 6135238..a9656c6 100644
--- a/programs/x509/cert_app.c
+++ b/programs/x509/cert_app.c
@@ -161,7 +161,7 @@
     memset(&cacrl, 0, sizeof(mbedtls_x509_crl));
 #endif
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;
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;
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 3e134dd..8a652fe 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -273,7 +273,7 @@
     mbedtls_x509_crt_init(&issuer_crt);
     memset(buf, 0, sizeof(buf));
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;
diff --git a/programs/x509/crl_app.c b/programs/x509/crl_app.c
index f06987c..d74a488 100644
--- a/programs/x509/crl_app.c
+++ b/programs/x509/crl_app.c
@@ -70,7 +70,7 @@
      */
     mbedtls_x509_crl_init(&crl);
 
-    if (argc == 0) {
+    if (argc < 2) {
 usage:
         mbedtls_printf(USAGE);
         goto exit;
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;