Fix core dump on Solaris 10 when "." isn't readable.

Fix core dump on Solaris 10 when "." isn't readable.

Reported by Peter Kruse in
<http://lists.gnu.org/archive/html/bug-tar/2013-10/msg00017.html>.
This doesn't fix all the Solaris 10 test failures, just the core dump.
* src/common.h, src/misc.c (tar_getcdpath): Now static.
* src/misc.c (normalize_filename): Report a fatal error
if cdpath is null, since we don't know the absolute name
of the working directory in that case.  FIXME: there should
be no need to know absolute file names.
(chdir_arg): Simplify wd allocation.
Don't assume that xgetcwd returns non-null.