Fix possible crash with merge callback

In some scenarios, the merge callback can be cleared sooner than when it
was initially set, tripping up the assertion we do in unbind().

Signed-off-by: Andrew Gunnerson <[email protected]>

Author: chenxiaolong

app/src/main/java/com/chiller3/custota/updater/UpdaterThread.kt

@@ -167,8 +167,15 @@ class UpdaterThread(
         // This cannot be unbound. The callback is removed from update_engine only when the merge
         // operation finishes or the binder client process dies.
         EngineCallback(EngineWatchType.MERGE).let {
-            updateEngine.cleanupSuccessfulUpdate(it)
+            // We set mergeCallback first because calling cleanupSuccessfulUpdate() may result in a
+            // synchronous call to onPayloadApplicationComplete(), which clears the callback.
             mergeCallback = it
+            try {
+                updateEngine.cleanupSuccessfulUpdate(it)
+            } catch (e: Exception) {
+                mergeCallback = null
+                throw e
+            }
         }
     }