efi_loader: Fix potential deref-after-null

After having been compared to a NULL value at efi_disk.c:426,
pointer 'part_info' is dereferenced at efi_disk.c:534.

Signed-off-by: Maks Mishin <maks.mishinFZ@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/lib/efi_loader/efi_disk.c b/lib/efi_loader/efi_disk.c
index 1f3de0a..5452640 100644
--- a/lib/efi_loader/efi_disk.c
+++ b/lib/efi_loader/efi_disk.c
@@ -531,7 +531,8 @@
 
 	/* Store first EFI system partition */
 	if (part && efi_system_partition.uclass_id == UCLASS_INVALID) {
-		if (part_info->bootable & PART_EFI_SYSTEM_PARTITION) {
+		if (part_info &&
+		    part_info->bootable & PART_EFI_SYSTEM_PARTITION) {
 			efi_system_partition.uclass_id = desc->uclass_id;
 			efi_system_partition.devnum = desc->devnum;
 			efi_system_partition.part = part;