Fix a possible NULL pointer dereference of ap_runtime_dir_relative()

ap_runtime_dir_relative() will return NULL on failure. However cgid_init()
does not check the return value of ap_runtime_dir_relative() and use it
directly.

Fix this bug by adding a NULL check.

Submitted by: Zhou Qingyang <zhou1615@umn.edu>

Github: closes #304


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1908972 13f79535-47bb-0310-9956-ffa450edef68
