bug#25387: guile-2.2 multi-thread segfault in SCM_VALIDATE_WEAK_TABLE

[Linas Vepstas]

Hi Andy: I just code-reviewed, it looks like a good fix;  you're saying that
the dynamic state was being accidentally collected when it shouldn't
have been.

Tested, it tests OK, after 40 mins cpu time, its still running.

--linas