Fix: cast $blog_id to int in wp_cache_clear_cache() to prevent crash on non-multisite installs

[DeveloperBatuhanALGUL]

Problem

When a third-party theme or plugin calls wp_cache_clear_cache('all', false) with a non-integer $blog_id, the function passes that value directly to get_supercache_dir(), which internally calls get_blog_option() — a multisite-only function. On non-multisite installs, this causes a fatal error.

Fixes #1019

Fix

Cast $blog_id to int at the top of the function. This ensures any non-integer value (e.g. 'all', false) is normalized to 0, which correctly triggers the single-site cache clearing path.

Testing

1. On a non-multisite WordPress install, call wp_cache_clear_cache('all', false)
2. Confirm no fatal error occurs and cache is cleared correctly