pp_sort.c: fix fencepost error in call to av_extend()

In [rt.cpan.org #39196] issue #17496 there is a report
that Tie::File produced spurious blank lines in the file
after

    @tied= sort @tied;

it turns out that this is because Tie::File treats
EXTEND similarly to STORESIZE (which is arguably not
entirely correct, but also not that weird) coupled
with an off by one error in the calls to av_extend()
in pp_sort.

This patch fixes the fencepost error, adds some comments
to av_extend() to make it clear what it is doing, and
adds a test that EXTEND is called by this code with
correct argument.

1 files changed, 3 insertions, 2 deletions

diff --git a/pp_sort.c b/pp_sort.c
index 0c5efb0869..4f81aaab7e 100644
--- a/pp_sort.c
+++ b/pp_sort.c
@@ -1067,7 +1067,8 @@ PP(pp_sort)
             for (i = 0; i < max; i++)
                 base[i] = newSVsv(base[i]);
             av_clear(av);
-            av_extend(av, max);
+            if (max)
+                av_extend(av, max-1);
             for (i=0; i < max; i++) {
                 SV * const sv = base[i];
                 SV ** const didstore = av_store(av, i, sv);
@@ -1094,7 +1095,7 @@ PP(pp_sort)
             }
             av_clear(av);
             if (max > 0) {
-                av_extend(av, max);
+                av_extend(av, max-1);
                 Copy(base, AvARRAY(av), max, SV*);
             }
             AvFILLp(av) = max - 1;