Problem: udp_engine_t fd cannot be init to NULL

Solution: initialize to -1, since fd is a file descriptor (int).

src/udp_engine.cpp

@@ -48,7 +48,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 zmq::udp_engine_t::udp_engine_t() :
     plugged (false),
-    fd(NULL),
+    fd(-1),
     session(NULL),
     handle(NULL),
     address(nullptr),