This was detected in about 25% of runs with gcc -fsanitize=address

  ERROR: AddressSanitizer: global-buffer-overflow on address ...
  READ of size 4 at 0x000000416628 thread T0
    #0 0x40479f in genpattern src/shred.c:782
    coreutils#1 0x4050d9 in do_wipefd src/shred.c:921
    coreutils#2 0x406203 in wipefile src/shred.c:1175
    coreutils#3 0x406b84 in main src/shred.c:1316
    coreutils#4 0x7f3454a1ef9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    coreutils#5 0x4025d8 (/tmp/coreutils-8.23/src/shred+0x4025d8)
  0x000000416628 is located 56 bytes to the left of
  global variable '*.LC49' from 'src/shred.c' (0x416660) of size 17
  0x000000416628 is located 12 bytes to the right of
  global variable 'patterns' from 'src/shred.c' (0x416540) of size 220
  SUMMARY: AddressSanitizer: global-buffer-overflow src/shred.c:782

* src/shred.c (gen_patterns): Restrict pattern selection
to the K available, which regressed due to v5.92-1462-g65533e1.
* tests/misc/shred-passes.sh: Add a deterministic test case.
* NEWS: Mention the bug fix.
Fixes http://bugs.gnu.org/20998

This was detected with:
  echo a > a; pr "-S$(printf "\t\t\t")" a -m a > /dev/null
Resulting in ASAN triggering:
  ====================================================
  ERROR: AddressSanitizer: global-buffer-overflow
  READ of size 1 at 0x00000041b622 thread T0
    #0 0x40506a in print_sep_string ../src/pr.c:2241
    #1 0x407ec4 in read_line ../src/pr.c:2493
    #2 0x40985c in print_page ../src/pr.c:1802
    #3 0x40985c in print_files ../src/pr.c:1618
    #4 0x4036e0 in main ../src/pr.c:1136

* src/pr.c (init_parameters): Ensure we only override the
specified separator when it's a single tab, thus matching
the calculated separator length.
* tests/pr/pr-tests.pl: Add a test case.
* NEWS: Mention the fix.