Fix a segfault with --internal-diff when no diff-cmd is set in the config.

This was introduced in r1464478, prior to that the NULL value would have
been handled in expand_option_value().  

* subversion/libsvn_subr/config.c
  (make_string_from_option): Check opt->value for NULL before using.

Suggested by: steveking


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467807 13f79535-47bb-0310-9956-ffa450edef68