Problem: all tests fail with assert in in_event

Solution: socket_base_t::in_event cannot do anything useful with
return status of process_commands. Asserting is the wrong solution,
as it is entirely valid to be interrupted or for the context to be
terminated, so discard the value.

src/socket_base.cpp

@@ -1503,9 +1503,8 @@ void zmq::socket_base_t::in_event ()
     if (thread_safe)
         reaper_signaler->recv();
 
-    int rc = process_commands (0, false);
+    process_commands (0, false);
     EXIT_MUTEX();
-    errno_assert(rc == 0);
     check_destroy();
 }