Trailing slash in theme Domain Path header causes double slashes in path

[bradyvercher]

The Domain Path header in themes is used to load the theme's translation files in WP_Theme::load_textdomain() when it is inactive.

The default value is /languages without a trailing slash and is appended to the theme directory, which is then prepended to the MO file in load_theme_textdomain().

If the Domain Path header has a trailing slash (ex: /languages/), the path ends up with double slashes. I'm not sure it's actually documented anywhere that the header shouldn't have a trailing slash, many of the popular themes in the directory use a trailing slash, and even the Automattic ​VIP Scanner plugin requires it to have one.

The load_plugin_textdomain() trims trailing slashes in various places to account for this. The attached patch does the same for load_theme_textdomain().

[johnbillion]

In 29984:

Avoid double path separators in load_theme_textdomain(). Fixes #29122. Props bradyvercher