Return an error when no name is parsed

When less than 1 RDN is successfully parsed in
mbedtls_x509_string_to_names(), return an error. Previously this
returned success when a string containing neither '=' or ',' was
supplied.

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/x509_create.c b/library/x509_create.c
index 50db956..cdfc82a 100644
--- a/library/x509_create.c
+++ b/library/x509_create.c
@@ -125,7 +125,7 @@
 
 int mbedtls_x509_string_to_names(mbedtls_asn1_named_data **head, const char *name)
 {
-    int ret = 0;
+    int ret = MBEDTLS_ERR_X509_INVALID_NAME;
     const char *s = name, *c = s;
     const char *end = s + strlen(s);
     const char *oid = NULL;
@@ -177,6 +177,9 @@
 
             s = c + 1;
             in_tag = 1;
+
+            /* Successfully parsed one name, update ret to success */
+            ret = 0;
         }
 
         if (!in_tag && s != c + 1) {