decode-bad-stack test failing (non-det) with "instr_encode_error: operands invalid"

AFAIK nobody has ever seen this before:

https://travis-ci.org/DynamoRIO/dynamorio/jobs/204498268

```
93/263 Test #92: code_api|security-common.decode-bad-stack ........................***Failed Required regular expression not found.Regex=[^Bad instr about to happen
Got an illegal instruction
$
] 0.38 sec
Bad instr about to happen

```

Start by reproducing the code_api|security-common.decode-bad-stack test from the linked Travis CI job and compare its expected output with the reported instr_encode error. Trace the failing non-deterministic path from the test output into the x86 instruction encoding code mentioned in the log; done means the test reliably reaches its expected illegal-instruction output and passes.

Written by the indexing model from the issue text.