IO buffer lifetime fix, source buffer must stay alive until the file write completes.

fdbclient/BackupContainerLocalDirectory.actor.cpp

@@ -63,7 +63,7 @@ public:
 		m_buffer = Standalone<VectorRef<uint8_t>>(old.slice(size, old.size()));
 
 		// Write the old buffer to the underlying file and update the write offset
-		Future<Void> r = holdWhile(old, m_file->write(old.begin(), size, m_writeOffset));
+		Future<Void> r = uncancellable(holdWhile(old, m_file->write(old.begin(), size, m_writeOffset)));
 		m_writeOffset += size;
 
 		return r;