Fix a double-free condition when byterange requests are made on brigades

Fix a double-free condition when byterange requests are made on brigades
containing any bucket that cannot be copied natively (ie, pipe or socket
buckets).

Before, we were reading that bucket to morph it to a heap bucket and then
taking the str that heap bucket points to and placing it in a second,
completely separate heap bucket.  That means we'd have two apr_bucket/
apr_bucket_heap pairs each with a refcount of 1 (rather than two apr_buckets
and a single apr_bucket_heap with a refcount of 2).  str would then be
doubly-freed when the second of those two buckets was destroyed.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90648 13f79535-47bb-0310-9956-ffa450edef68