Bugfix: postponed sanity check of (bordeaux-threads:join-thread) until a thread is actually started.
Partially fixes bug [5](https://github.com/cosmos72/stmx/issues/5) on ECL.

Not a complete fix due to a known ECL bug triggered with bordeaux-threads 0.8.3, see:
http://permalink.gmane.org/gmane.lisp.bordeaux-threads.devel/212
http://sourceforge.net/p/ecls/bugs/261/

The remaining issue is:
-------------------------------------------------------------------------------
There is no problem when bordeaux-threads is compiled
for the first time, however thereafter loading from fasls will result in

    Condition of type: BORDEAUX-MP-CONDITION
    There is no support for this method on this implementation.
-------------------------------------------------------------------------------