util: Alter virAuthGet*Path API to check valid parameters

Before trying to dereference @auth, let's ensure it's valid.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NMarcos Paulo de Souza <marcos.souza.org@gmail.com>

src/util/virauth.c

@@ -153,6 +153,12 @@ virAuthGetUsernamePath(const char *path,
     if (ret != NULL)
         return ret;
 
+    if (!auth) {
+        virReportError(VIR_ERR_INVALID_ARG, "%s",
+                       _("Missing authentication credentials"));
+        return NULL;
+    }
+
     memset(&cred, 0, sizeof(virConnectCredential));
 
     if (defaultUsername != NULL) {
@@ -226,6 +232,12 @@ virAuthGetPasswordPath(const char *path,
     if (ret != NULL)
         return ret;
 
+    if (!auth) {
+        virReportError(VIR_ERR_INVALID_ARG, "%s",
+                       _("Missing authentication credentials"));
+        return NULL;
+    }
+
     memset(&cred, 0, sizeof(virConnectCredential));
 
     if (virAsprintf(&prompt, _("Enter %s's password for %s"), username,