maint: avoid "definitely lost" valgrind warnings

Since commit v8.23-19-g8defcee, main() will return,
rather than call exit(), this inducing "definitely lost"
warnings in valgrind's leak checker.  That precludes using
the following to flag memory leaks:

  valgrind --leak-check=full --error-exitcode=1 \
           --errors-for-leak-kinds=definite

* src/pr.c (main): In dev builds, explicitly free memory allocated.
* src/sort.c (main): Likewise.
* src/tail.c (main): Likewise.
* src/tsort.c (tsort): Likewise.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/pr.c b/src/pr.c
index d79d84f18..887e12584 100644
--- a/src/pr.c
+++ b/src/pr.c
@@ -1130,6 +1130,7 @@ main (int argc, char **argv)
     }
 
   cleanup ();
+  IF_LINT (free (file_names));
 
   if (have_read_stdin && fclose (stdin) == EOF)
     error (EXIT_FAILURE, errno, _("standard input"));