area of implicit nil is not enough to nregs

There is a possibility that protrude from the stack.

def foo
  "a" < 2
end

irep 00C3A1A8 nregs=4 nlocals=2 pools=1 syms=1 reps=0
000 OP_ENTER    0:0:0:0:0:0:0
001 OP_STRING   R2      "a"
002 OP_LOADI    R3      2
003 OP_LT       R2      :<      1
004 OP_RETURN   R2

R2 ...receiver
R3 ...Argument
R4 ...Block(nil)

nregs 5 need.