MB-38673: clear error_extra_json in Cookie::reset
MB-38673: clear error_extra_json in Cookie::reset

Spotted in testing that the error_extra_json from a previous failure
was included in a subsequent failure.

Change-Id: I1f8809b1162d6f6b38a43bd9698ba53f4dbb7d84
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/125954
Reviewed-by: Paolo Cocchi <9dd88c9f3e5cbab6e19a6c020f107e0c648ac956@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine