[Bug #1670] assignment to $: breaks rb_vm_struct

Bug #1670: assignment to $: breaks rb_vm_struct
http://redmine.ruby-lang.org/issues/show/1670

e$B5/I<<Te(B: Yuki S.
e$B%9%F!<%?%9e(B: Open, e$BM%@hEYe(B: Normal
e$BC4Ev<Te(B: Yusuke E., e$B%+%F%4%je(B: core, Target version: 1.9.2
ruby -v: ruby 1.9.2dev (2009-06-21 trunk 23774) [i386-darwin9.7.0]

$:e$B$Ne(Bsettere$B$,%G%U%)%k%H$Ne(Bsettere$B$J$N$G!"e(Brb_vm_structe$B$Ne(Bselfe$B$r>e=q$-$7$^$9!#e(B

1.8e$B$KJo$&$He(B$:e$B$Oe(Breadonlye$B$G$"$k$Y$-$G$9!#e(B

e$B%A%1%C%He(B #1670 e$B$,99?7$5$l$^$7$?!#e(B (by Yusuke E.)

e$B%9%F!<%?%9e(B Opene$B$+$ie(BClosede$B$KJQ99e(B
e$B?JD=e(B % 0e$B$+$ie(B100e$B$KJQ99e(B

Applied in changeset r23809.

http://redmine.ruby-lang.org/issues/show/1670