[1.9] VM::InstructionSequence#disasm

 This patch introduce Qundef

        * compile.c (compile_cpath): use Qundef to denote cbase lookup.

 but ruby don't like it

vgs% ./ruby -ve 'puts VM::InstructionSequence.new("class A; end").disasm'
ruby 1.9.0 (2008-05-05 revision 16293) [i686-linux]
-e:1: [BUG] Segmentation fault
ruby 1.9.0 (2008-05-05 revision 16293) [i686-linux]

-- control frame ----------
c:0004 p:---- s:0010 b:0010 l:000009 d:000009 CFUNC  :disasm
c:0003 p:0024 s:0007 b:0006 l:000005 d:000005 TOP    -e:1
c:0002 p:---- s:0004 b:0004 l:000003 d:000003 FINISH :private_class_method
c:0001 p:0000 s:0002 b:0002 l:000001 d:000001 TOP    <dummy toplevel>:17
---------------------------
DBG> : "-e:1:in `<main>'"
-- backtrace of native function call (Use addr2line) --
0x8118e54
0x813d652
0x813d69b
0x80db7b9
0xb7f54440
0x80605af
0x8086d65
0x8109f0f
0x810a1b2
0x810ab0f