Introduce ap_debug_assert() macro, like ap_assert() but only active if

Introduce ap_debug_assert() macro, like ap_assert() but only active if
AP_DEBUG is defined.

ap_get_client_block():
. avoid some cases where we leak a temporary bucket brigade
. clean up/fix the logic to copy a brigade into the caller's buffer;
  the wrong length was used in some cases
. add an AP_DEBUG-only assertion for some assumptions made regarding
  the brigade returned by the filters

Submitted by:	partly by Greg Stein, but of course anything bad is mine

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86583 13f79535-47bb-0310-9956-ffa450edef68