Check string is nonempty before indexing

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

Resolves #1238. An unchecked indexing operation on an empty string introduced by #1228 causes a crash if compiled with -Wp,-D_GLIBCXX_ASSERTIONS, and possibly under other conditions

Intended Effect:

Resolves the crash

How to Verify:

Side Effects:

Documentation:

Copyright and Licensing

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By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[WardBrian]

@serban-nicusor-toptal is there an easy way to set -Wp,-D_GLIBCXX_ASSERTIONS in our CI? Seems like it would catch things like this earlier.

[serban-nicusor-toptal]

I think the easiest way is to add that at the end of writeFile here https://github.com/stan-dev/cmdstan/blob/develop/Jenkinsfile#L11