Throw away unexpected data received by XPUB sockets, rather than asserting.

Fixes LIBZMQ-310.
Signed-off-by: Staffan Gimåker <staffan@spotify.com>

src/xpub.cpp

@@ -63,7 +63,7 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
         //  Apply the subscription to the trie.
         unsigned char *data = (unsigned char*) sub.data ();
         size_t size = sub.size ();
-        zmq_assert (size > 0 && (*data == 0 || *data == 1));
+        if (size > 0 && (*data == 0 || *data == 1)) {
             bool unique;
             if (*data == 0)
                 unique = subscriptions.rm (data + 1, size - 1, pipe_);
@@ -76,6 +76,7 @@ void zmq::xpub_t::xread_activated (pipe_t *pipe_)
                 pending.push_back (blob_t ((unsigned char*) sub.data (),
                     sub.size ()));
         }
+    }
 }
 
 void zmq::xpub_t::xwrite_activated (pipe_t *pipe_)