Don't crash for empty errinfo

Change-Id: Iabf9b26bbd000bf9ed408cf37bbdf21f6cd8b17c
Reviewed-on: http://review.couchbase.org/24352
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>