Added missing close method w/ check if socket is already closed.

Signed-off-by: Guido Goldstein <github@a-nugget.de>

include/zmq.hpp

@@ -214,8 +214,7 @@ namespace zmq
 
         inline ~socket_t ()
         {
-            int rc = zmq_close (ptr);
-            assert (rc == 0);
+            close();
         }
 
         inline operator void* ()
@@ -223,6 +222,17 @@ namespace zmq
             return ptr;
         }
 
+        inline void close()
+        {
+            if(ptr == NULL)
+                // already closed
+                return ;
+            int rc = zmq_close (ptr);
+            if (rc != 0)
+                throw error_t ();
+            ptr = 0 ;
+        }
+
         inline void setsockopt (int option_, const void *optval_,
             size_t optvallen_)
         {