func_strings: fix for memmove patch test

In r404674 the AST_TEST_DEFINE(test_REPLACE) test was added
that made use of a function that doesn't exist in 1.8. This
fixes that by reverting to directly accessing chan varshead.

Reported by: Tzafrir Cohen
(issue ASTERISK-22910)



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@404951 f38db490-d61c-443f-a65b-d21fe96a405b