pr: fix infinite loop when double spacing

* src/pr.c (init_parameters): Ensure we avoid a 0 lines_per_body
which was possible when adjusting for double spacing.
That caused print_page() to always return true,
causing an infinite loop.
* tests/pr/pr-tests.pl: Add a test case.
* NEWS: Mention the fix.
Fixes https://bugs.debian.org/1034808

NEWS

@@ -14,6 +14,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   factor, numfmt, and tsort now diagnose read errors on the input.
   [This bug was present in "the beginning".]
 
+  'pr --length=1 --double-space' no longer enters an infinite loop.
+  [This bug was present in "the beginning".]
+
 ** Changes in behavior
 
   'cp -v' and 'mv -v' will no longer output a message for each file skipped

src/pr.c

@@ -1209,7 +1209,7 @@ init_parameters (int number_of_files)
     lines_per_body = lines_per_page;
 
   if (double_space)
-    lines_per_body = lines_per_body / 2;
+    lines_per_body = MAX (1, lines_per_body / 2);
 
   /* If input is stdin, cannot print parallel files.  BSD dumps core
      on this. */

tests/pr/pr-tests.pl

@@ -415,6 +415,9 @@ my @tv = (
 ['padding2', '-t -n,64', "1\n", (" "x 63)."1,1\n", 0],
 # Ensure we handle buffer truncation correctly
 ['padding3', '-t -N1000000 -n,1', "1\n", "0,1\n", 0],
+
+# This entered an infinite loop before coreutils-9.4
+['page-length1', '-dl1', "", "", 0],
 );
 
 # Convert the above old-style test vectors to the newer