gcc: fix a couple of gcc warning (sign compare)

My gosh, they're like rabbit :p
pcsx2: improve the GS read fifo API

1/ initReadFifo will be first called on the GS thread
    (openGL can only be done on the GS thread)

2/ readFifo will be called on the EE thread. It is not safe too access eeMem from GS
 because of memory is virtual

Fix "recent" regression (crash) on Kingdom heart and others game too.

v2: add a len check on GSState::InitReadFIFO