pr: fix read from invalid memory with tabs in separator

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.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/pr/pr-tests.pl b/tests/pr/pr-tests.pl
index 4d85dc989..ec3980af3 100755
--- a/tests/pr/pr-tests.pl
+++ b/tests/pr/pr-tests.pl
@@ -467,6 +467,13 @@ push @Tests,
     {IN=>{3=>"x\ty\tz\n"}},
      {OUT=>join("\t", qw(a b c m n o x y z)) . "\n"} ];
 
+# This resulted in reading invalid memory before coreutils-8.26
+push @Tests,
+   ['asan1', "-m -S'\t\t\t' -t",
+    {IN=>{1=>"a\n"}},
+    {IN=>{2=>"a\n"}},
+     {OUT=>"a\t\t\t\t  \t\t\ta\n"} ];
+
 @Tests = triple_test \@Tests;
 
 my $save_temps = $ENV{DEBUG};