Disabled redirects to named locations if URI is not set.

Disabled redirects to named locations if URI is not set.

If something like "error_page 400 @name" is used in a configuration,
a request could be passed to a named location without URI set, and this
in turn might result in segmentation faults or other bad effects
as most of the code assumes URI is set.

With this change nginx will catch such configuration problems in
ngx_http_named_location() and will stop request processing if URI
is not set, returning 500.