libsnapshot: Fix intermittent test failure due to missing null check.

Bug: 152547190
Test: vts_snapshot_test
Change-Id: If7adc4fba61325e52d6268531ce0a15b00c0a380
(cherry picked from commit 533b2b62965a6ce3509b07fefc178232547fe58d)
Merged-In: If7adc4fba61325e52d6268531ce0a15b00c0a380
diff --git a/fs_mgr/libsnapshot/snapshot.cpp b/fs_mgr/libsnapshot/snapshot.cpp
index 1eec6a4..b825805 100644
--- a/fs_mgr/libsnapshot/snapshot.cpp
+++ b/fs_mgr/libsnapshot/snapshot.cpp
@@ -2505,7 +2505,9 @@
     }
 
     UpdateState state = ProcessUpdateState([&]() -> bool {
-        callback();
+        if (callback) {
+            callback();
+        }
         return true;
     });
     LOG(INFO) << "Update state in recovery: " << state;