etdump: Fix the file name included in the could not open error message

If only one file is passed, this reads the terminating NULL and so
prints (null). If multiple files are passed then this always prints the
second file, which may or may not exist (and will be particularly
confusing if it does exist since the output will include the dump of the
second file).

MFC after:	1 week

(cherry picked from commit 340cebe990)

usr.bin/etdump/etdump.c

@@ -262,7 +262,7 @@ main(int argc, char **argv)
 		} else {
 			iso = fopen(argv[i], "r");
 			if (iso == NULL)
-				err(1, "could not open %s", argv[1]);
+				err(1, "could not open %s", argv[i]);
 		}
 		dump_eltorito(iso, argv[i], outfile, outputter);
 	}