Fix use of 'NULL' instead of 'nil' in Lua code.

src/poller.nobj.lua

@@ -400,7 +400,7 @@ int zmq_poll(zmq_pollitem_t *items, int nitems, long timeout);
 	-- did we find a pending event?
 	if(idx >= 0) then
 		-- push the event's sock/fd.
-		if(${this}.items[idx].socket ~= NULL) then
+		if(${this}.items[idx].socket ~= nil) then
 			sock = obj_type_ZMQ_Socket_push(${this}.items[idx].socket, 0)
 		else
 			sock = tonumber(${this}.items[idx].fd)

src/pre_generated-zmq.nobj.c

@@ -1601,7 +1601,7 @@ static const char zmq_ffi_lua_code[] = "-- try loading luajit's ffi\n"
 "	-- did we find a pending event?\n"
 "	if(idx >= 0) then\n"
 "		-- push the event's sock/fd.\n"
-"		if(this.items[idx].socket ~= NULL) then\n"
+"		if(this.items[idx].socket ~= nil) then\n"
 "			sock = obj_type_ZMQ_Socket_push(this.items[idx].socket, 0)\n"
 "		else\n"
 "			sock = tonumber(this.items[idx].fd)\n"