Fix for undefined offset warning in do_action

[chrisjean]

An unchecked index in the do_action function found in wp-includes/plugin.php can cause a huge flood of warnings depending on data feed to it.

I simply added an isset check to ensure that the use of the index doesn't occur unless something is at that array location.

[chrisbliss18]

This plugin enables warnings and produces the warning at the top of the screen.

[chrisbliss18]

This problem is still in trunk as of r13402. I've added a plugin that shows how this warning can be produced. It's basically an unchecked potential $arg value that causes this.

The following criteria must be met for this warning to be produced:

• The called action tag must have at least one function attached to it.
• The second argument passed to the do_action function (the $arg value) must be an array with only one entry with an index other than 0.

When these criteria is met, the is_object($arg[0]) portion of the code throws a warning since the conditional has not confirmed that $arg[0] exists first.

My patch simply confirms that $arg[0] is set before checking it, thus removing the warning.

[westi]

I can't reproduce the notice but I see no harm in the change.

[westi]

(In [13674]) Fix a possible undefined offset warning in do_action. Fixes #11241 props chrisbliss18.