Fix Assertion Failure on Selecting Camera Column

[shun-iwasawa]

This PR will resolve assertion failure occurs with the following steps in debug mode:

1. Select Animate tool.
2. Select camera column.

The assertion failure occurs at line 212 of tstageobject.cpp ;

const TStageObjectId TStageObjectId::ColumnId(int index) {
  assert(0 <= index && index <= StageObjectMaxIndex);
  return TStageObjectId(COLUMN << StageObjectTypeShift | index);
}

There were cases that the function TStageObjectId::ColumnId() is called with the argument index being -1 when the camera column is selected.

[RodneyBaker]

LGTM