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

Props kovshenin
fixes #24272


git-svn-id: http://core.svn.wordpress.org/trunk@24198 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove unused variables. props ocean90. fixes #24247.

git-svn-id: http://core.svn.wordpress.org/trunk@24199 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add missing semicolon. props jcastaneda. fixes #24282.

git-svn-id: http://core.svn.wordpress.org/trunk@24200 1a063a9b-81f0-0310-95a4-ce76da25c4cd