efi_loader: HandleProtocol parameter checks

HandleProtocol() and OpenProtocol() have to return EFI_UNSUPPORTED if the
protocol is not installed on the handle.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_loader/efi_boottime.c b/lib/efi_loader/efi_boottime.c
index ab299aa..6d86daf 100644
--- a/lib/efi_loader/efi_boottime.c
+++ b/lib/efi_loader/efi_boottime.c
@@ -2636,8 +2636,15 @@
 	}
 
 	r = efi_search_protocol(handle, protocol, &handler);
-	if (r != EFI_SUCCESS)
+	switch (r) {
+	case EFI_SUCCESS:
+		break;
+	case EFI_NOT_FOUND:
+		r = EFI_UNSUPPORTED;
+		goto out;
+	default:
 		goto out;
+	}
 
 	r = efi_protocol_open(handler, protocol_interface, agent_handle,
 			      controller_handle, attributes);