Handle NULL pointer as good as possible (Coverity #50099)
As pointed by Coverity, in the "Configuration" panel of WPrefs there are
some images loaded, but if the images cannot be loaded correctly then the
returned NULL pointer can crash the application as it is dereferenced in
further function calls.
To solve this case, this patch is adding a NULL pointer check in the
functions RScaleImage (wrlib) and WMCreatePixmapFromRImage (WINGs), so both
can accept that NULL pointer to also return NULL, which means the existing
check for "icon == NULL" in the WPrefs code will be useful.
Signed-off-by: Christophe CURIS <christophe.curis@free.fr>