Avoid notice with search feed without orderby

[nacin]

A conditional assumes that 'orderby' is set as a query variable, but that's only fair to assume when we're not looking at feeds. Turns out it isn't a query variable that is filled, so we'll need to add an isset check.

This would be minor but a displayed notice could screw up feed parsing.

[azaozz]

Works as expected, +1.

[nacin]

In 25889:

Avoid notice with search feeds.

fixes #25677.

[nacin]

In 25890:

Avoid notice with search feeds.

Merges [25889] to the 3.7 branch.

fixes #25677.