Problem: in case of exhausted resources on creation of a context, assertions are triggered

Solution: signal error to caller, and apply appropriate cleanup

src/mailbox.hpp

@@ -54,6 +54,8 @@ namespace zmq
         void send (const command_t &cmd_);
         int recv (command_t *cmd_, int timeout_);
 
+        bool valid () const;
+
 #ifdef HAVE_FORK
         // close the file descriptors in the signaller. This is used in a forked
         // child process to close the file descriptors so that they do not interfere