Fixing #isTerminated

[dupriezt]

The current #isTerminated method does not seem to work.

Running the following in a playground raises a "#isRoot was sent to nil" exception because the execution has reached the root context and is asking whether its parent (nil) is root to decide whether the execution is terminated.

interpreter := DASTInterpreter new.
interpreter initializeWithProgram: (RBParser parseExpression: 'Point new').
[ interpreter isTerminated  ] whileFalse: [ interpreter stepInto ].

This pull request make #isTerminated simply check whether the current context is the root context. With it, the example above runs without signalling an exception.