util: removed unnecessary RReleaseImage in wmsetbg (Coverity #50077)

As pointed by Coverity, it is not useful to call RReleaseImage if we are in
the branch of code where we know that the image for which it is being
called is NULL.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

util/wmsetbg.c

@@ -646,7 +646,6 @@ static BackgroundTexture *parseTexture(RContext * rc, char *text)
 
 		if (!gradient) {
 			wwarning("could not render texture:%s", RMessageForError(RErrorCode));
-			RReleaseImage(gradient);
 			RReleaseImage(image);
 			goto error;
 		}