defined?(@x) returns nil for nil value

```
% ruby -e '@x = nil; p defined?(@x); p defined?(@y)'
nil
nil
```

```
% multiruby -e '@x = nil; p defined?(@x); p defined?(@y)'

VERSION = mri_trunk
CMD     = ~/.multiruby/install/mri_trunk/bin/ruby -e @x = nil; p defined?(@x); p defined?(@y)

"instance-variable"
nil

RESULT = 0

VERSION = 1.8.7-p249
CMD     = ~/.multiruby/install/1.8.7-p249/bin/ruby -e @x = nil; p defined?(@x); p defined?(@y)

"instance-variable"
nil

RESULT = 0

VERSION = 1.9.2-p320
CMD     = ~/.multiruby/install/1.9.2-p320/bin/ruby -e @x = nil; p defined?(@x); p defined?(@y)

"instance-variable"
nil

RESULT = 0

VERSION = 1.9.3-p194
CMD     = ~/.multiruby/install/1.9.3-p194/bin/ruby -e @x = nil; p defined?(@x); p defined?(@y)

"instance-variable"
nil

RESULT = 0

TOTAL RESULT = 0 failures out of 4

Passed: 1.9.2-p320, 1.8.7-p249, mri_trunk, 1.9.3-p194
Failed: 
```