message: Make empty label names fail syntax checking
  Previously, writing a single ':' on a line would cause the program to crash,
  given that no checks were made to check whether the contents left of ':'
  actually contained any label definition.
diff: @@ -623,6 +623,9 @@ QString SyntaxHighlighter::checkSyntax(const QString& input) {
  // Label detected - check if already defined, else add to label definitions
  QStringList splitFirst = string.split(':');
  string = splitFirst[0]; // get label
  + if (string.isEmpty()) {
  + return QString("Empty label name");
  + }
  // Update map entries at given block
  if (m_labelPosMap.contains(string) && m_labelPosMap[string] != pos) {
  // duplicate label found
  