Minor patch to keep ICC compiler happy

ICC doesn't recognise that assert(false) terminates the program
and thus complains that certain functions have no return values.
This patch supplies dummy return values to keep the compiler happy.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/msg.cpp

@@ -188,6 +188,7 @@ void *zmq::msg_t::data ()
         return u.lmsg.content->data;
     default:
         zmq_assert (false);
+        return NULL;
     }
 }
 
@@ -203,6 +204,7 @@ size_t zmq::msg_t::size ()
         return u.lmsg.content->size;
     default:
         zmq_assert (false);
+        return 0;
     }
 }