Handle a potential null value

I don't understand how this happens, but:
* It's uncommon
* It's been reported once during startup
* I'm assuming this is a race condition during startup
* Let's just treat the scroll position as zero if unset

Then let's cross our fingers and hope we don't crash anywhere else in
this situation.

Fixes: #198

m/scrollPosition.go

@@ -386,7 +386,12 @@ func numberPrefixLength(pager *Pager, scrollPosition scrollPositionInternal) int
 	// recursion.
 	//
 	// Let's improve on demand.
-	maxVisibleLineNumber := scrollPosition.lineNumber.NonWrappingAdd(
+	var lineNumber linenumbers.LineNumber
+	// Ref: https://github.com/walles/moar/issues/198
+	if scrollPosition.lineNumber != nil {
+		lineNumber = *scrollPosition.lineNumber
+	}
+	maxVisibleLineNumber := lineNumber.NonWrappingAdd(
 		scrollPosition.deltaScreenLines +
 			pager.visibleHeight() - 1)
 	if maxVisibleLineNumber.IsAfter(maxPossibleLineNumber) {