Fix FTBFS and crash triggered by <AppSystem>.load_from_desktop_file()

- Avoid error '"iconname" may be used uninitialized in this function'
  by initializing said variable to NULL.

- Define shell_util_get_file_description as static (like the other
  similar functions) to avoid another compiler error.

- Don't save errors from g_key_file_load_from_data_dirs into the
  variable "error" (ie. pass NULL to it instead). Without this,
  gnome-shell crashes if the key file can't be found (with message
  "Error invoking Shell.load_from_desktop_file: Valid key file could
  not be found in search dirs").

- Check the result of the load_from_desktop_file() call in places.js,
  as it may be null.

src/shell-app-system.c

@@ -652,7 +652,7 @@ shell_app_system_load_from_desktop_file (ShellAppSystem   *system,
     {
       char *app_path = g_build_filename ("applications", filename, NULL);
       success = g_key_file_load_from_data_dirs (keyfile, app_path, &full_path,
-                                                G_KEY_FILE_NONE, error);
+                                                G_KEY_FILE_NONE, NULL);
       g_free (app_path);
     }
 
@@ -784,7 +784,7 @@ shell_app_info_get_desktop_file_path (ShellAppInfo *info)
 GIcon *
 shell_app_info_get_icon (ShellAppInfo *info)
 {
-  char *iconname;
+  char *iconname = NULL;
   GIcon *icon;
 
   /* This code adapted from gdesktopappinfo.c

src/shell-uri-util.c

@@ -106,7 +106,7 @@ shell_util_get_file_display_for_common_files (GFile *file)
   return NULL;
 }
 
-char *
+static char *
 shell_util_get_file_description (GFile *file)
 {
   GFileInfo *info;