tar: if (p) free (p); -> free (p);

There is no longer (since SunOS 4) any need to guard against
free (NULL), so replace each "if (p) free (p);" with "free (p);".
From Jim Meyering in
<http://lists.gnu.org/archive/html/bug-tar/2011-01/msg00026.html>.
* src/incremen.c (scan_directory, read_directory_file): As above.
(try_purge_directory): Likewise.
* src/list.c (read_header): Likewise.
* src/misc.c (assign_string): Likewise.