efi_loader: check length in CreateDeviceNode()

When creating a device path node ensure that the size of the allocated
memory at lest matches the size of the node header.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_loader/efi_device_path.c b/lib/efi_loader/efi_device_path.c
index 6104c7d..10f890f 100644
--- a/lib/efi_loader/efi_device_path.c
+++ b/lib/efi_loader/efi_device_path.c
@@ -335,6 +335,9 @@
 {
 	struct efi_device_path *ret;
 
+	if (length < sizeof(struct efi_device_path))
+		return NULL;
+
 	ret = dp_alloc(length);
 	if (!ret)
 		return ret;