media: vim2m: don't use curr_ctx->dev before checking

It seems that it is possible that dev to be null, as there's
a warning printing:
	"Instance released before the end of transaction"

Solves this warning:
	drivers/media/platform/vim2m.c: drivers/media/platform/vim2m.c:525 device_work() warn: variable dereferenced before check 'curr_ctx' (see line 523)
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/platform/vim2m.c

@@ -520,13 +520,14 @@ static void device_work(struct work_struct *w)
 	unsigned long flags;
 
 	curr_ctx = container_of(w, struct vim2m_ctx, work_run.work);
-	vim2m_dev = curr_ctx->dev;
 
 	if (NULL == curr_ctx) {
 		pr_err("Instance released before the end of transaction\n");
 		return;
 	}
 
+	vim2m_dev = curr_ctx->dev;
+
 	src_vb = v4l2_m2m_src_buf_remove(curr_ctx->fh.m2m_ctx);
 	dst_vb = v4l2_m2m_dst_buf_remove(curr_ctx->fh.m2m_ctx);