swrast: Fix memory leak.

Fixes resource leak defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/drivers/dri/swrast/swrast.c

@@ -363,6 +363,7 @@ swrast_new_renderbuffer(const struct gl_config *visual, __DRIdrawable *dPriv,
 	xrb->bpp = 8;
 	break;
     default:
+	free(xrb);
 	return NULL;
     }