sanitize_bookmark_field() assumes link_category is an array

[mdawaffe]

A link object's link_category property is an array of the link category ids to which the link belongs. In rare circumstances, though, it can be set to a scalar.

Attached casts the link_category value to an array before calling array_map(). This avoids a PHP warning.

Patch also optimizes the conditional logic a bit by turning the series of $field if statements into a single switch statement and by moving the esc_attr/esc_js logic into the only $context branch to which it applies.

[ryan]

(In [14239]) Fix array handling in sanitize_bookmark_field(). Props mdawaffe. fixes #13098

[nacin]

Related, maybe duplicate (see comments on dbDelta): #12854.