Make "cb_str_postprocess()" work like "cb_wstr_postprocess()" by using

Make "cb_str_postprocess()" work like "cb_wstr_postprocess()" by using
"tvb_get_string()" rather than "tvb_format_text()".  (This fixes a
problem wherein we freed the return value of "tvb_format_text()", which
we should not do as "tvb_format_text()" doesn't dynamically allocate the
buffer it returns, but means that we don't handle non-printable
characters - but that's a problem with "cb_wstr_postprocess()" as well,
as "tvb_fake_unicode()" also doesn't handle non-printable characters
specially; we should fix the problem in *both* routines.)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@7955 f5534014-38df-0310-8fa8-9805f1628bb7