Fix unallocated memory read in Markup parser

[PhilArmstrong]

Small memory read error found whilst fuzzing another application that uses the Swift Markup parser.

[CodaFi]

Fix LGTM, do you have a test case?

[PhilArmstrong]

Not for Swift. I'll see if I can come up with something - I think it should be triggered by parsing "///\n" under ASAN though.

[PhilArmstrong]

Can you suggest a way to get any of the command line tools to parse the doc comments? I can't seem to find a way to do it.

[CodaFi]

I believe test/SourceKit/MarkupXML/basic.swift wanders its way through this code path. If not, trace your way out from the Markdown parser to the requisite SourceKit request (here it's markup-xml) and search for related tests.

[PhilArmstrong]

If you run the tests with ASAN compiled binaries then the existing test file: test/IDE/comment_brief.swift will trigger this bug.

(It does so when I run the test command from the lit file at the command line at least.)

[PhilArmstrong]

As will any documentation on a function that looks like:

\\\
func greet(person: String) -> String {
...

[CodaFi]

Then let’s give this a go

@swift-ci please ASAN test

[CodaFi]

@swift-ci please smoke test and merge