This issue causes a failure of TestTracePid that was observed in CI:
https://travis-ci.com/github/go-delve/delve/jobs/343053383
I'm not sure what causes it in this particular instance but there are
several ways in which a thread stopped at a breakpoint might have a
BreakpointInfo == nil field (see variable withBreakpointInfo in
debugger.Debugger.Command).