Attachment medata can evaluate as truth, but still be corrupted

[nbachiyski]

At two places in the code there are checks like:

if ( !$imagedata = wp_get_attachment_metadata( $post_id ) )

which if satisfied expect $imagedata to be an array. But if there is non-unserializable value in the postmeta table it will be returned as a string, which will break the code after the check.

Here is patch, which uses is_array() instead.

(In [8261]) Make sure attachment meta data is an array. Props nbachiyski. fixes #7252