Fix overflow when creating OW value from string
Fix overflow when creating OW value from string

For OW, OL, OV, OF, and OD the value constructor allows pointers
of any numerical type and simply does a memcpy.  If the type of
the pointer was less than the value type size, then the allocated
space could be less than the memcpy, causing a buffer overlow.
This fix ensures that the memcpy size is the size of the buffer.

dgobbi/vtk-dicom,dgobbi/vtk-dicom,dgobbi/vtk-dicom