[CM] Recent s7 broken on MSVC

[bil at ccrma.Stanford.EDU]

Thanks very much for the bug report.  If this is a recent
version of s7 (see S7_DATE in s7.h), I think the problem is
that the MSVC version of the code calls unbound_variable
where everyone else returns NULL.  I don't have access to
a machine running MSVC, and I can't remember now why the
two cases are different.  I'll make a patch that I think
will fix this.  Thanks again.