mythuibuttonlist: Prevent reading beyond end of list

Prevent a potential SEGV in MythUIButtonList::SetPosition by testing
if an iterator is < end() before dereferencing.

This was picked up by valgrind.

Signed-off-by: Lawrence Rust <lvr@softsystem.co.uk>
Signed-off-by: Stuart Morgan <smorgan@mythtv.org>