Fix a crash if you don't specify 'lpxserver'

luprex/cpp/drv/driver.cpp

@@ -632,6 +632,12 @@ class Driver {
             }
         }
 
+        // Make sure there's exactly one argument left for the engine type.
+        if (argc != 1) {
+            std::cerr << "Must specify the engine type" << std::endl;
+            return 1;
+        }
+
         // Initialize state variables.
         read_console_recently_ = false;
         chbuf_.reset(new char[DRV_SHORTSTRING_SIZE]);