Fix incorrect group calc state check.

And the code in the else block is pretty dangerous as it passes an empty
token array to the group interpreter whose effect is entirely unpredictable.

Also, declare the destructor of CompiledFormula class to be virtual as it
serves as a base class for DynamicKernel.

Change-Id: I4e191550a4437ad6ebac55fcdeee4f3654722ff1