check_connected(): fix leak of pack-index mmap

Since c6807a40dc (clone: open a shortcut for connectivity check,
2013-05-26), we may open a one-off packed_git struct to check what's in
the pack we just received. At the end of the function we throw away the
struct (rather than linking it into the repository struct as usual).

We used to leak the struct until dd4143e7bf (connected.c: free the
"struct packed_git", 2022-11-08), which calls free(). But that's not
sufficient; inside the struct we'll have mmap'd the pack idx data from
disk, which needs an munmap() call.

Building with SANITIZE=leak doesn't detect this, because we are leaking
our own mmap(), and it only finds heap allocations from malloc(). But if
we use our compat mmap implementation like this:

  make NO_MMAP=MapsBecomeMallocs SANITIZE=leak

then LSan will notice the leak, because now it's a regular heap buffer
allocated by malloc().

We can fix it by calling close_pack(), which will free any associated
memory. Note that we need to check for NULL ourselves; unlike free(), it
is not safe to pass a NULL pointer to close_pack().

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

connected.c

@@ -159,6 +159,9 @@ no_promisor_pack_found:
 		err = error_errno(_("failed to close rev-list's stdin"));
 
 	sigchain_pop(SIGPIPE);
-	free(new_pack);
+	if (new_pack) {
+		close_pack(new_pack);
+		free(new_pack);
+	}
 	return finish_command(&rev_list) || err;
 }