Dr. Memory internal crash at PC 0x643af49b

This was reported in http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5052/steps/memory%20test%3A%20extensions/logs/stdio:

```
[ RUN ] SerialApiTest.StashAndRestoreDuringEcho

~~Dr.M~~ WARNING: application exited with abnormal code 0xffffffff
```

There are other similar failures in http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5050/steps/memory%20test%3A%20extensions/logs/stdio, http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5051/steps/memory%20test%3A%20extensions/logs/stdio, http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5053/steps/memory%20test%3A%20extensions/logs/stdio, http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5043/steps/memory%20test%3A%20extensions/logs/stdio, http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5011/steps/memory%20test%3A%20extensions/logs/stdio, and http://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Unit%20%28DrMemory%20full%29%20%285%29/builds/5063/steps/memory%20test%3A%20extensions/logs/stdio.