Fix potential crash in mod_authz_svn during startup if 
path specified by AuthzSVNAccessFile, AuthzSVNReposRelativeAccessFile, or 
AuthzSVNGroupsFile directives is not accessible.

* subversion/mod_authz_svn/mod_authz_svn.c
  (canonicalize_access_file): Check return value of 
   ap_server_root_relative() all and return NULL on error.
  (AuthzSVNAccessFile_cmd, AuthzSVNReposRelativeAccessFile_cmd, 
   AuthzSVNGroupsFile_cmd): Check return value canonicalize_access_file() 
   call.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541432 13f79535-47bb-0310-9956-ffa450edef68