msOGRLayerGetAutoStyle passes NULL pointer to GetRGBFromString causing access violation

[szekerest]

msOGRLayerGetAutoStyle may pass NULL pointer to GetRGBFromString causing access
violation.

In my test case the brush style in the OGR MapInfo tab file contains ForeColor
but does not contain BackColor style. In this case
poBrushStyle->BackColor(bIsNull) returns NULL and bIsNull = true;

To fix the problem the following code fragment is used:

pszColor = poBrushStyle->BackColor(bIsNull);
                  if (!bIsNull &&        
poBrushStyle->GetRGBFromString(pszColor,r,g,b,t))
    {
        MS_INIT_COLOR(c->styles[0].backgroundcolor, r, g, b);
    }