Remove duplicated code.

Fixes CID #965941.
Fix an incorrect cast.

The return value was casted instead of dwFrames.

Fixes CID #965944.
Avoid useless conversion to CStringA.

Fixes CID #1111768.
CVobFile: Avoid hiding methods without overriding them.

Fixes CID #965828 #965829.
Check some return values when it makes sense to do so.

Fixes CID #1111761 #1111762.
Improve the Get(Cur)DispMode and SetDispMode functions.

Fixes CID #966222.