If a plugin/theme changes AtomParser::FILE to an invalid or unavailable
resource, fopen will return `false`.

- PHP 8: `fread` throws a fatal error as its 1st param requires
  a resource type.
- <PHP 8: `fread` and `fclose` throw a warning, but code continues
  to run.

This commit:

- wraps the entire `while` block and `fclose` to prevent the PHP 8
  fatal error.
- allows the remaining code to run to ensure the parser is freed
  and error handler restored.