Table::prepareColumns does not properly validate type

[2easy]

Function Table::prepareColumns does not validate if Table::generateColumn returns NULL pointer so the following line:
columns[i] -> setColumnId(i);
throws segfault.

Suggested solution: validate if generateColumn is returning NULL and if so, throw an exception

[przemekpastuszka]

Validation story has not yet been completed, so this is expected behaviour. Please, add any validation requests to #12