decode-bad-stack is flaky, attempts to encode illegal instruction

_From [rnk@google.com](https://code.google.com/u/rnk@google.com/) on October 16, 2012 16:17:16_

I don't understand this at all, but I'm filing it since we're aiming to get the suite green.

Example failing build: http://build.chromium.org/p/client.drmemory/builders/linux-lucid_x64-dr/builds/1107 Test output:
60: Bad instr about to happen
60:
60:

Start with the failing security-common.decode-bad-stack suite test and compare the reported Linux build 1107 output with a local run. Read dynamorio/core/x86/encode.c around line 2341, where instr_encode reports that no encoding was found. Done means the test no longer flakes and the illegal-instruction case is handled without the reported encoding error.

Written by the indexing model from the issue text.