Fix position of marker for dereference of nullable errors

[TonyxSun]

Fix position of marker for DEREFERENCE_OF_NULLABLE type errors.

Previous behavior:

// Class.java:10: error: [dereference.of.nullable] dereference of possibly-null reference firstObj.intList
//         firstObj.intList.add(1);
//                 ^

New behavior:

// Class.java:10: error: [dereference.of.nullable] dereference of possibly-null reference firstObj.intList
//         firstObj.intList.add(1);
//                         ^

[TonyxSun]

Dependent on eisop/checker-framework.demos#3. Lost context as it is too old. Feel free to close