Fix Coverity resource leak complaint.

Coverity is convinced there's a resource leak in the code path through __wt_metadata_search:

__wt_metadata_search ->

    __wt_turtle_read ->

        __metadata_config ->

            __wt_config_collapse ->

                __wt_strndup

and variants of this complaint are currently 25% of our Coverity issues.

Try and convince Coverity there's really no problem here.