Fix recursive calling of QScriptProgram

The first time a QScriptProgram is evaluated, it gets compiled and
then executed. If the execution would somehow trigger another
evaluation of the script program, it would run into the compile
stage again (even though it already was compiled), and then trigger
and assertion in debug mode (or leak memory in release builds).

Task-number: QTBUG-37317
Change-Id: I83e7efd5f238d021e200258826e2e4a9520c3a7d
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>