xxd: do not try reading from closed stdin when non-existent file is given

Was:
	xxd: FILE: No such file or directory
	xxd: FILE: Bad file descriptor

function                                             old     new   delta
next                                                 276     278      +2

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/dump.c

@@ -357,6 +357,7 @@ static NOINLINE int next(priv_dumper_t *dumper)
 				if (!freopen(fname, "r", stdin)) {
 					bb_simple_perror_msg(fname);
 					dumper->exitval = 1;
+					dumper->next__done = 1;
 					continue;
 				}
 			}