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

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

src/lib/commands/xdump.c

@@ -142,7 +142,7 @@ xdprval(struct valstr *val, char *sep)
 	    pr("%snil", sep);
 	break;
     default:
-	CANT_HAPPEN("Bad VAL type");
+	CANT_REACH();
 	pr("0");
     }
     return " ";