boot: fix vbe_find_first_device()

uclass_find_first_device() may return NULL if no device for the uclass
exists. Handle this case gracefully.

Addresses-Coverity: CID 356244 ("Null pointer dereferences (FORWARD_NULL)")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/vbe.c b/boot/vbe.c
index e6ee087..52b3283 100644
--- a/boot/vbe.c
+++ b/boot/vbe.c
@@ -40,7 +40,7 @@
 int vbe_find_first_device(struct udevice **devp)
 {
 	uclass_find_first_device(UCLASS_BOOTMETH, devp);
-	if (*devp && is_vbe(*devp))
+	if (!*devp || is_vbe(*devp))
 		return 0;
 
 	return vbe_find_next_device(devp);