message: fix backspace spell bug
diff: @@ -347,6 +347,7 @@ class Speller {
  let segoff = this.sharedString.client.mergeTree.getContainingSegment(pos,
  MergeTree.UniversalSequenceNumber, this.sharedString.client.getClientId());
  + if (segoff && segoff.segment) {
  if (segoff.offset !== 0) {
  console.log("expected pos only at segment boundary");
  }
  @@ -388,6 +389,7 @@ class Speller {
  this.sharedString.annotateRange({ textError: textErrorInfo }, start, end);
  } else {
  if (this.verbose) {
  + // tslint:disable:max-line-length
  console.log(`spell ok (${start}, ${end}): ${words.substring(result.index, re.lastIndex)}`);
  }
  this.sharedString.annotateRange({ textError: null }, start, end);
  @@ -397,6 +399,7 @@ class Speller {
  while (result);
  }
  }
  + }
  private initSpellerService(intelligence: IIntelligentService, queryString: string, startPos: number) {
  if (this.serviceCounter < Speller.spellerParagraphs) {
  