Followup to r1573096, handle possible error case when getting the
CFMutableDataRef.

* subversion/libsvn_subr/sysinfo.c
  (system_version_plist): Check for NULL and bail.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573101 13f79535-47bb-0310-9956-ffa450edef68