fs_mgr: Clear AvbOps struct.

This ensures that future operations (e.g. function pointers in the
AvbOps struct) added to AvbOps are set to NULL.

Bug: 37709309
Test: Manually tested on UEFI-based bootloader.
Change-Id: If83ea57b7abad94e472768c594efa210e3351a4d
diff --git a/fs_mgr/fs_mgr_avb_ops.cpp b/fs_mgr/fs_mgr_avb_ops.cpp
index edcfd54..caee4ec 100644
--- a/fs_mgr/fs_mgr_avb_ops.cpp
+++ b/fs_mgr/fs_mgr_avb_ops.cpp
@@ -96,7 +96,9 @@
     // We only need to provide the implementation of read_from_partition()
     // operation since that's all what is being used by the avb_slot_verify().
     // Other I/O operations are only required in bootloader but not in
-    // user-space so we set them as dummy operations.
+    // user-space so we set them as dummy operations. Also zero the entire
+    // struct so operations added in the future will be set to NULL.
+    memset(&avb_ops_, 0, sizeof(AvbOps));
     avb_ops_.read_from_partition = read_from_partition;
     avb_ops_.read_rollback_index = dummy_read_rollback_index;
     avb_ops_.validate_vbmeta_public_key = dummy_validate_vbmeta_public_key;