fix linked list corruption in flockfile lists

commit 5345c9b8 added a linked list to
track the FILE streams currently locked (via flockfile) by a thread.
due to a failure to fully link newly added members, removal from the
list could leave behind references which could later result in writes
to already-freed memory and possibly other memory corruption.

implicit stdio locking was unaffected; the list is only used in
conjunction with explicit flockfile locking.

this bug was not present in any releases; it was introduced and fixed
during the same release cycle.

patch by Timo Teräs, who discovered and tracked down the bug.

src/stdio/ftrylockfile.c

@@ -34,6 +34,7 @@ int ftrylockfile(FILE *f)
 	f->lockcount = 1;
 	f->prev_locked = 0;
 	f->next_locked = self->stdio_locks;
+	if (f->next_locked) f->next_locked->prev_locked = f;
 	self->stdio_locks = f;
 	return 0;
 }