logd: add locks around ~SerializedFlushToState

This accesses shared resources in SerializedLogBuffer and therefore
requires a lock.

Bug: 169736426
Test: malloc_debug_system_tests
Change-Id: I807c65f4719481f933b4917a50f83f933b1929fb
diff --git a/logd/SerializedLogBuffer.cpp b/logd/SerializedLogBuffer.cpp
index fa90878..6d1576f 100644
--- a/logd/SerializedLogBuffer.cpp
+++ b/logd/SerializedLogBuffer.cpp
@@ -201,6 +201,11 @@
     return std::make_unique<SerializedFlushToState>(start, log_mask);
 }
 
+void SerializedLogBuffer::DeleteFlushToState(std::unique_ptr<FlushToState> state) {
+    auto lock = std::unique_lock{lock_};
+    state.reset();
+}
+
 bool SerializedLogBuffer::FlushTo(
         LogWriter* writer, FlushToState& abstract_state,
         const std::function<FilterResult(log_id_t log_id, pid_t pid, uint64_t sequence,