core: Do not over allocate memory within 'ap_rgetline_core' for the common case.

trunk patch: http://svn.apache.org/r1483005
Submitted by: jailletc36
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1485728 13f79535-47bb-0310-9956-ffa450edef68