Bail on computing the amount of bytes in a brigade if any of the buckets

have indeterminate length (e.g., a pipe bucket). Passing an invalid length
into mod_mem_cache can cause it to overflow its cache buffer.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93698 13f79535-47bb-0310-9956-ffa450edef68