threadpool: do not propagate exceptions through the dtor

This would call terminate.
We ignore exceptions in pthread_join instead, as this is not
a fatal problem here.

Coverity 182568

src/common/threadpool.cpp

@@ -57,7 +57,8 @@ threadpool::~threadpool() {
     has_work.notify_all();
   }
   for (size_t i = 0; i<threads.size(); i++) {
-    threads[i].join();
+    try { threads[i].join(); }
+    catch (...) { /* ignore */ }
   }
 }