gallery_shortcode() can throw a "trying to get property of non-object" notice if called without a post context

[kovshenin]

The gallery_shortcode() function uses $post->ID without checking whether get_post() returned an actual post object. If called when a post context is not set, it throws a notice. To reproduce call this inside the ​Debug Bar Console:

do_shortcode( '[gallery ids="1,2,3"]' );

[ryan]

In 24198:

Avoid warning in gallery_shortcode() when called outside of post context.

Props kovshenin
fixes #24272

[SergeyBiryukov]

#20870 was marked as a duplicate.