gpulib: fix out-of-bounds reads in do_cmd_buffer()

gpulib: fix out-of-bounds reads in do_cmd_buffer()

When gpu.cmd_buffer[] is filling up, and the last 1 or 2 words
in it are the beginning of a new vram read/write cmd, do_cmd_buffer()
would access out-of-bounds, reading garbage pos/size data.

Fixes corrupted gfx in this PS1 .exe test utility:
https://github.com/PeterLemon/PSX/tree/master/CPUTest/CPU/LOADSTORE/LW
(This and all similar tests on Peter's site).
Note that gfx access in this utility is done entirely through cmds given
through GPUwriteData(), i.e. direct CPU->GP0 stores, not DMA.