etnaviv: fix a null pointer dereference

This issue was found by cppcheck

Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>

src/etnaviv/drm/etnaviv_bo.c

@@ -238,11 +238,11 @@ out_unlock:
 /* destroy a buffer object */
 void etna_bo_del(struct etna_bo *bo)
 {
-	struct etna_device *dev = bo->dev;
-
 	if (!bo)
 		return;
 
+	struct etna_device *dev = bo->dev;
+
 	if (!p_atomic_dec_zero(&bo->refcnt))
 		return;