ReHighlightStates: sanity-check startline value (#3237)

Check if startline value is valid before passing it to input.State(),
to prevent a theoretically possible race when the number of lines
changes in the meantime, causing an out of bounds access.

Actually this race cannot happen: ReHighlightStates() is only called
from the main goroutine, and the line array is modified, again, only by
the main goroutine. So for now this change is rather cosmetic: it is
just to make the highligher API implementation self-sufficiently safe
without assumptions about which goroutines are using which API functions
and how.

pkg/highlight/highlighter.go

@@ -363,7 +363,9 @@ func (h *Highlighter) ReHighlightStates(input LineStates, startline int) int {
 	h.lastRegion = nil
 	if startline > 0 {
 		input.Lock()
-		h.lastRegion = input.State(startline - 1)
+		if startline-1 < input.LinesNum() {
+			h.lastRegion = input.State(startline - 1)
+		}
 		input.Unlock()
 	}
 	for i := startline; ; i++ {