Buildbot/MTR: crashes callstack is not saved in some cases

Sometimes,on buildbot when crashes happen " during shutdown", the crash callstacks are lost

1. stdio log is saved by buildbot, and contains notification that crash happened, but no callstack.

2.error log that had callstack is not saved.

Example : http://buildbot.askmonty.org/buildbot/builders/win64-release2/builds/8146/

The only informaton we have about crash is the below snipppet from stdio log
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***Warnings generated in error logs during shutdown after running tests: main.shm

151117 21:26:50 [ERROR] mysqld got exception 0xc0000005 ;
Attempting backtrace. You can use the following information to find out
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