(CANT_REACH): New. Use it instead of CANT_HAPPEN() where possible.

(oops): Cope with NULL argument passed by CANT_REACH().

src/lib/commands/prod.c

@@ -296,7 +296,7 @@ prod(void)
 		pr(" %4.0f ", real);
 		break;
 	    default:
-		CANT_HAPPEN("bad TYPE");
+		CANT_REACH();
 		pr("  ??? ");
 		break;
 	    }