Check product exists before accessing methods

To avoid fatal errors when generating renewal orders in Subscriptions.
Merge pull request #4902 from thenbrent/master

Check product exists before accessing methods when sending order email
Update abstract-wc-product.php

Add filter to is_taxable() method so that plugins can better control what is and isn't taxable based on various conditions, such as customer's billing or shipping address, etc.
Load frontend lang first, and remove no-longer needed alt_lang loader Closes #4874
remove un-wanted hentry from products Closes #4880
Passing post_id to the coupon save action
Pass post_id to the woocommerce_coupon_options_save action so we know what
coupon ID is being saved.
cancel_order - take no action if already cancelled.
Prevent ambiguous column error
Merge pull request #4924 from paulvandermeijs/develop-fix_exclude_protected_products

Prevent ambiguous column error
Limit item name lengths Closes #4922
For custom attributes, try to turn back to 'name' from saved slug in emails/view order templates Closes #4833
Workaround relevanssi Closes #4873
Merge pull request #4912 from whatthefork/master

Update abstract-wc-product.php
Use parseHTML and enforce 1.8 jquery Closes #4899
Ensure button text stays changed
Layered nag - remember orderby #4889
Merge pull request #4919 from ChuckMac/Coupon_action

Passing post_id to the coupon save action
Fix variation level reporting Closes #4885
using wordpress brace code style in flat rate shipping
commenting regex in flat rate shipping