proc: bugfix: onNextGoroutine and breakpoints with nil condition

Next will add internal breakpoints with nil condition if it can't find
the current goroutine (possibly because there isn't a current goroutine
because the runtime hasn't been initialized yet).
onNextGoroutine should skip breakpoints with nil condition, otherwise
we'll end up with an internal debugger error trying to walk a nil
expression.

Updates #893

pkg/proc/threads.go

@@ -407,7 +407,7 @@ func onRuntimeBreakpoint(thread Thread) bool {
 func onNextGoroutine(thread Thread, breakpoints map[uint64]*Breakpoint) (bool, error) {
 	var bp *Breakpoint
 	for i := range breakpoints {
-		if breakpoints[i].Internal() {
+		if breakpoints[i].Internal() && breakpoints[i].Cond != nil {
 			bp = breakpoints[i]
 			break
 		}