MFIFO: Just over the top of the ring buffer is not classed as in the buffer (reaching the top is masked anyway).  Fixes DDR X and Supernova games. 

Bug was introduced between r4034 and r4039.

git-svn-id: http://pcsx2.googlecode.com/svn/trunk@5584 96395faa-99c1-11dd-bbfe-3dabce05a288