ref-utils: Make remote name validation public and take an error

We should be consistent with the validation in the session-helper, so
let's make this validation function available to it.

Helps: https://github.com/flatpak/flatpak/security/advisories/GHSA-v2gw-v9h5-9q4x

common/flatpak-ref-utils-private.h

@@ -33,6 +33,9 @@ gboolean flatpak_is_valid_branch (const char *string,
 gboolean flatpak_is_valid_arch (const char *string,
                                 gssize      len,
                                 GError    **error);
+gboolean flatpak_is_valid_remote_name (const char *string,
+                                       gssize      len,
+                                       GError    **error);
 gboolean flatpak_has_name_prefix (const char *string,
                                   const char *name);
 gboolean flatpak_name_matches_one_wildcard_prefix (const char         *string,

common/flatpak-ref-utils.c

@@ -567,25 +567,42 @@ is_valid_remote_name_character (gint c)
     c == '.';
 }
 
-static gboolean
-is_valid_remote_name (const char *remote,
-                      gsize len)
+gboolean
+flatpak_is_valid_remote_name (const char *string,
+                              gssize      len,
+                              GError    **error)
 {
   const char *end;
 
-  if (len == 0)
-    return FALSE;
+  if (len < 0)
+    len = strlen (string);
 
-  end = remote + len;
-
-  if (!is_valid_initial_remote_name_character (*remote++))
-    return FALSE;
-
-  while (remote < end)
+  if (G_UNLIKELY (len == 0))
     {
-      char c = *remote++;
-      if (!is_valid_remote_name_character (c))
-        return FALSE;
+      flatpak_fail_error (error, FLATPAK_ERROR_INVALID_NAME,
+                          _("Remote name can't be empty"));
+      return FALSE;
+    }
+
+  end = string + len;
+
+  if (G_UNLIKELY (!is_valid_initial_remote_name_character (*string)))
+    {
+      flatpak_fail_error (error, FLATPAK_ERROR_INVALID_NAME,
+                          _("Remote name can't start with %c"), *string);
+      return FALSE;
+    }
+  string++;
+
+  while (string < end)
+    {
+      if (G_UNLIKELY (!is_valid_remote_name_character (*string)))
+        {
+          flatpak_fail_error (error, FLATPAK_ERROR_INVALID_NAME,
+                              _("Remote name can't contain %c"), *string);
+          return FALSE;
+        }
+      string++;
     }
 
   return TRUE;
@@ -622,7 +639,7 @@ _flatpak_decomposed_new (char            *ref,
       const char *colon = strchr (p, ':');
       if (colon != NULL)
         {
-          if (!is_valid_remote_name (ref, colon - ref))
+          if (!flatpak_is_valid_remote_name (ref, colon - ref, NULL))
             {
               flatpak_fail_error (error, FLATPAK_ERROR_INVALID_REF, _("Invalid remote name"));
               return NULL;