Fix array bounds error in ImfMultiPartInputFile.
Fix array bounds error in ImfMultiPartInputFile.

oglops/PyIlmBase,oglops/PyIlmBase,oglops/PyIlmBase,oglops/IlmBase,oglops/IlmBase,oglops/OpenEXR,oglops/OpenEXR,oglops/IlmBase,oglops/OpenEXR,oglops/PyIlmBase