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.

NEWS

@@ -35,6 +35,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   nl now resets numbering for each page section rather than just for each page.
   [This bug was present in "the beginning".]
 
+  pr now handles specified separator strings containing tabs correctly.
+  Previously it would have output random data from memory.
+  [This bug was detected with ASAN and present in "the beginning".]
+
   sort -h -k now works even in locales that use blank as thousands separator.
 
   stty --help no longer outputs extraneous gettext header lines

src/pr.c

@@ -1233,7 +1233,7 @@ init_parameters (int number_of_files)
         }
       /* It's rather pointless to define a TAB separator with column
          alignment */
-      else if (!join_lines && *col_sep_string == '\t')
+      else if (!join_lines && col_sep_length == 1 && *col_sep_string == '\t')
         col_sep_string = column_separator;
 
       truncate_lines = true;

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};