fix test 0 #24, use PN_ZERO, fix DBG_vt,DBG_vi

replace PN_NUM(0) by PN_ZERO (less work for cc)
fix unary with binary TEST on p2, since 0 is false on p2, but true on potion
  and the binary repr of 0 was 1. add PN_TEST1 for the unary test.
add one more jit stack corruption check
improve example/fib.p2
fix the broken logic for DBG_vt,DBG_vi