r600g: Don't blindly unmap NULL->size.

There may actually be something mapped in that range, especially for large
buffers like e.g. the GL Drawable.

src/gallium/winsys/r600/drm/radeon_bo.c

@@ -145,7 +145,9 @@ struct radeon_bo *radeon_bo_decref(struct radeon *radeon, struct radeon_bo *bo)
 		return NULL;
 	}
 
-	munmap(bo->data, bo->size);
+	if (bo->map_count) {
+		munmap(bo->data, bo->size);
+	}
 	memset(&args, 0, sizeof(args));
 	args.handle = bo->handle;
 	drmIoctl(radeon->fd, DRM_IOCTL_GEM_CLOSE, &args);