OpenGL graphics crash

[John W. Eaton]

On  8-Jul-2009, Michael Goffioul wrote:

| The function convert_cdata computes a real RGB version of CDATA
| property. If CDATA is already in RGB format, it returns it, otherwise
| it maps the CDATA values to the figure colormap according to the
| SCALED property.
| 
| That being said, I'm also unsure about what should happen when
| CDATA contains NA elements and what's the purpose of having
| such values in CDATA...

OK.

The following change avoids the crash and the slice demo looks correct
to me.

  http://hg.savannah.gnu.org/hgweb/octave/rev/d8d410b08228

jwe