overlapFunc will return RTGUI_REGION_STATUS_FAILURE in some
circumstance(OOM etc), check it and return error if any thing goes
wrong. It prevents garbage values crashing the system.
Please ignore the eol diff noise.
git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1802 bbd45198-f89e-11dd-88c7-29a3b14d5316