Check if the dumpdir read from the archive is well-formed before using it.

Check if the dumpdir read from the archive is well-formed before using it.

* src/common.h (dumpdir_ok): New proto.
* src/incremen.c (dumpdir_ok): Take size as the second argument.
Verify if the last byte is 0 and the dumpdir contains 0 or more
nul-terminated strings.
(get_gnu_dumpdir): Check if the obtained dumpdir is ok. Clear the
is_dumpdir flag if it is not.
* src/xheader.c (dumpdir_decoder) Verify if the obtained dumpdir is
ok.