Zero out 'globals' in the ASTInterpreter constructor

Maybe the GC should zero-initialize everything, but it doesn't,
so we would get garbage values in that field.

src/codegen/ast_interpreter.cpp

@@ -278,6 +278,7 @@ ASTInterpreter::ASTInterpreter(CompiledFunction* compiled_function)
       generator(0),
       edgecount(0),
       frame_info(ExcInfo(NULL, NULL, NULL)),
+      globals(0),
       frame_addr(0) {
 
     CLFunction* f = compiled_function->clfunc;