ab: Fix send length calculation

... for the non-blocking connect case introduced in 2.4.30. 

Submitted By: jorton
Reviewed By: ylavic, druggeri, covener




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1825839 13f79535-47bb-0310-9956-ffa450edef68