Fix issue 12537 pixelValueAt should be more robust

Fix issue https://pharo.fogbugz.com/f/cases/12537/pixelValueAt-primitive-can-be-more-robust

pixelValueAt: will not fail in case of hibernated Form, but rather answer wrong pixel value.
The primitive should fail and let image side cope with it (it is already equipped for handling the failure).
In any case, the size of bitmap must be checked for consistency.